21 Essential PHP Interview Questions – toptal.com

Consider the following code:

$str1 = 'yabadabadoo';
$str2 = 'yaba';
if (strpos($str1,$str2)) {
    echo "\"" . $str1 . "\" contains \"" . $str2 . "\"";
} else {
    echo "\"" . $str1 . "\" does not contain \"" . $str2 . "\"";

The output will be:

"yabadabadoo" does not contain "yaba"

Why? How can this code be fixed to work correctly?

The problem here is that strpos() returns the starting position index of $str1 in $str2 (if found), otherwise it returns false. So in this example, strpos() returns 0 (which is then coerced to false when referenced in the if statement). That’s why the code doesn’t work properly.

The correct solution would be to explicitly compare the value returned by strpos() to false as follows:

$str1 = 'yabadabadoo';
$str2 = 'yaba';
if (strpos($str1,$str2) !== false) {
    echo "\"" . $str1 . "\" contains \"" . $str2 . "\"";
} else {
    echo "\"" . $str1 . "\" does not contain \"" . $str2 . "\"";

Note that we used the !== operator, not just the != operator. If we use !=, we’ll be back to the problem that 0 is coerced to false when referenced in a boolean expression, so 0 != false will evaluate to false.

What will be the output of the code below and why?

$x = 5;
echo $x;
echo "<br />";
echo $x+++$x++;
echo "<br />";
echo $x;
echo "<br />";
echo $x---$x--;
echo "<br />";
echo $x;

What will be the values of $a and $b after the code below is executed? Explain your answer.

$a = '1';
$b = &$a;
$b = "2$b";

Both $a and $b will be equal to the string "21" after the above code is executed.

Here’s why:

The statement $b = &$a; sets $b equal to a reference to $a (as opposed to setting $b to the then-current value of $a). Thereafter, as long as $b remains a reference to $a, anything done to $a will affect $b and vice versa.

So when we subsequently execute the statement $b = "2$b", $b is set equal to the string "2" followed by the then-current value of $b (which is the same as $a) which is 1, so this results in $b being set equal to the string "21" (i.e., the concatenation of "2" and "1"). And, since $b is a reference to $a, this has the same affect on the value of $a, so both end up equal to "21".

What will be the output of each of the statements below and why?

var_dump(0123 == 123);
var_dump('0123' == 123);
var_dump('0123' === 123);

var_dump(0123 == 123) will output bool(false) because the leading 0 in 0123 tells the PHP interpreter to treat the value as octal (rather than decimal) value, and 123 octal is equal to 83 decimal, so the values are not equal.

var_dump('0123' == 123) will output bool(true) since the string 0123 will automatically be coerced to an integer when being compared with an integer value. Interestingly, when this conversion is performed, the leading 0 is ignored and the value is treated as a decimal (rather than octal) value, so the values are bother 123 (decimal) and are therefore equal.

var_dump('0123' === 123) outputs bool(false) since it performs a more strict comparison and does not do the automatic type coercion of the string to an integer.

What is the problem with the code below? What will it output? How can it be fixed?

$referenceTable = array();
$referenceTable['val1'] = array(1, 2);
$referenceTable['val2'] = 3;
$referenceTable['val3'] = array(4, 5);

$testArray = array();

$testArray = array_merge($testArray, $referenceTable['val1']);
$testArray = array_merge($testArray, $referenceTable['val2']);
$testArray = array_merge($testArray, $referenceTable['val3']);

The output will be as follows:

array(2) { [0]=> int(1) [1]=> int(2) }

You may also see two warnings generated, similar to the following:

Warning: array_merge(): Argument #2 is not an array
Warning: array_merge(): Argument #1 is not an array

The issue here is that, if either the first or second argument to array_merge() is not an array, the return value will be NULL. For example, although one might reasonably expect that a call such as array_merge($someValidArray, NULL) would simply return $someValidArray, it instead returns NULL! (And to make matters worse, this is not documented well at all in the PHP documentation.)

As a result, the call to $testArray = array_merge($testArray, $referenceTable['val2']) evaluates to $testArray = array_merge($testArray, 3) and, since 3 is not of type array, this call to array_merge() returns NULL, which in turn ends up setting $testArray equal to NULL. Then, when we get to the next call to array_merge(), $testArray is now NULL so array_merge() again returns NULL. (This also explains why the first warning complains about argument #2 and the second warning complains about argument #1.)

The fix for this is straightforward. If we simply typecast the second argument to an array, we will get the desired results. The corrected array_merge() calls would therefore be as follows:

$testArray = array_merge($testArray, (array)$referenceTable['val1']);
$testArray = array_merge($testArray, (array)$referenceTable['val2']);
$testArray = array_merge($testArray, (array)$referenceTable['val3']);

which will yield the following output (and no warnings):

array(2) { [0]=> int(1) [1]=> int(2) } 
array(3) { [0]=> int(1) [1]=> int(2) [2]=> int(3) } 
array(5) { [0]=> int(1) [1]=> int(2) [2]=> int(3) [3]=> int(4) [4]=> int(5) }

What will this code output and why?

$x = true and false;

Surprisingly to many, the above code will output bool(true) seeming to imply that the and operator is behaving instead as an or.

The issue here is that the = operator takes precedence over the and operator in order of operations, so the statement $x = true and false ends up being functionally equivalent to:

$x = true;       // sets $x equal to true
true and false;  // results in false, but has no affect on anything

This is, incidentally, a great example of why using parentheses to clearly specify your intent is generally a good practice, in any language. For example, if the above statement $x = true and false were replaced with $x = (true and false), then $x would be set to false as expected.

What will $x be equal to after the statement $x = 3 + "15%" + "$25"?

The correct answer is 18.

Here’s why:

PHP supports automatic type conversion based on the context in which a variable or value is being used.

If you perform an arithmetic operation on an expression that contains a string, that string will be interpreted as the appropriate numeric type for the purposes of evaluating the expression. So, if the string begins with one or more numeric characters, the remainder of the string (if any) will be ignored and the numeric value is interpreted as the appropriate numeric type. On the other hand, if the string begins with a non-numeric character, then it will evaluate to zero.

With that understanding, we can see that "15%" evaluates to the numeric value 15 and "$25" evaluates to the numeric value zero, which explains why the result of the statement $x = 3 + "15%" + "$25" is 18 (i.e., 3 + 15 + 0).

After the code below is executed, what will be the value of $text and what will strlen($text) return? Explain your answer.

$text = 'John ';
$text[10] = 'Doe';

After the above code is executed, the value of $text will be the string “John D” (i.e., “John”, followed by 5 spaces, followed by “D”) and strlen($text) will return 11.

There are two things going on here.

First of all, since $text is a string, setting a single element of $text simply sets that single character to the value specified. The statement $text[10] = 'Doe' therefore sets that single character to 'D' (i.e., the first character in the string "Doe", since an element of a string can only be a single character).

Secondly, $text[10] = 'Doe' says to set the 11th character of the string (remember that indices are zero-based) to 'D'. Prior to that statement, though, the length of the string $text ("John ") was only 5. Whereas compilers or interpreters in other languages might barf (with something akin to an out-of-bounds-index error) when you then attempt to set the 11th character of a 5 character string, PHP instead is very “accommodating” and instead allows this and sets all intermediate characters to blanks.

PHP_INT_MAX is a PHP constant that corresponds to the largest supported integer value (value is based on the version of PHP being run and the platform it is running on).

Assume that var_dump(PHP_INT_MAX) will yield int(9223372036854775807).

In that case, what will be the result of var_dump(PHP_INT_MAX + 1)? Also, what will be the result of var_dump((int)(PHP_INT_MAX + 1))?

NOTE: It’s not important to supply the exact value when answering the question, but rather to explain what will happen and why.

The result of var_dump(PHP_INT_MAX + 1) will be displayed as a double (in the case of this specific example, it will display double(9.2233720368548E+18)). The key here is for the candidate to know that PHP handles large integers by converting them to doubles (which can store larger values).

And interestingly, the result of var_dump((int)(PHP_INT_MAX + 1)) will be displayed as a negative number (in the case of this specific example, it will display int(-9223372036854775808)). Again, the key here is for the candidate to know that the value will be displayed as a negative number, not to know the precise value.

How would you sort an array of strings to their natural case-insensitive order, while maintaing their original index association?

For example, the following array:

	'0' => 'z1',
	'1' => 'Z10',
	'2' => 'z12',
	'3' => 'Z2',
	'4' => 'z3',

After sorting, should become:

	'0' => 'z1',
	'3' => 'Z2',
	'4' => 'z3',
	'1' => 'Z10',
	'2' => 'z12',

The trick to solving this problem is to use three special flags with the standard asort() library function:


The function asort() is a variant of the standard function sort() that preserves the index association. The three flags used above SORT_STRING, SORT_FLAG_CASE and SORT_NATURAL forces the sort function to treat the items as strings, sort in a case-insensitive way and maintain natural order respectively.

Note: Using the natcasesort() function would not be a correct answer, since it would not maintain the original index association of the elements of the array.

What is PEAR in php?

PEAR (PHP Extension and Application Repository) is a framework and repository for reusable PHP components. PEAR is a code repository containing all kinds of php code snippets and libraries.

PEAR also offers a command-line interface that can be used to automatically install packages.

What are the differences between echo and print in PHP?

echo and print are largely the same in PHP. Both are used to output data to the screen.

The only differences are as follows:

  1. echo does not return a value whereas print does return a value of 1 (this enables print to be used in expressions).
  2. echo can accept multiple parameters (although such usage is rare) while print can only take a single argument.

Consider the following code:

$x = NULL;

if ('0xFF' == 255) {
    $x = (int)'0xFF';

What will be the value of $x after this code executes? Explain your answer.

Perhaps surprisingly, the answer is neither NULL nor 255. Rather, the answer is that $x will equal 0 (zero).


First, let’s consider whether '0xFF' == 255 will evaluate to true or false. When a hex string is loosely compared to an integer, it is converted to an integer. Internally, PHP uses is_numeric_string to detect that the string contains a hex value and converts it to an integer (since the other operand is an integer). So in this case, ‘0xFF’ is converted to its integer equivalent which is 255. Since 255 = 255, this condition evaluates to true. (Note that this only works for hex strings, not for octal or binary strings.)

But if that’s the case, shouldn’t the statement $x = (int)'0xFF'; execute and result in $x being set equal to 255?

Well, the statement does execute, but it results in $x being set equal to 0, not 255 (i.e., it is not set to the integer equivalent of ‘0xFF’). The reason is that the explicit type cast of the string to an integer uses convert_to_long (which works differently than the is_numeric_string function that was used in evaluating the conditional expression, as explained above). convert_to_long processes the string one character at a time from left to right and stops at the first non-numeric character that it reaches. In the case of ‘0xFF’, the first non-numeric character is ‘x’, so the only part of the string processed is the initial ‘0’. As a result, the value returned by (int)'0xFF' is 0, so when the code completes, $x will be equal to 0.


What is use of the header() function in PHP?

  1. header() is used to redirect from one page to another: header("Location: index.php");
  2. header() is used to send an HTTP status code: header("HTTP/1.0 this Not Found");
  3. header() is used to send a raw HTTP header: header('Content-Type: application/json');

What is the difference between include_once() and require_once(), which one would you use in circumstances where you need to connect to a database, and why?

include_once() or include allows a file to be included, and in cases where the file is missing or has the wrong name, we receive an error message and execution will still continue regardless.

On the other hand, require_once() or require would be suitable in cases where a file needs to be included once and if it is missing or has a wrong name then we receive a fatal error and the execution of the program stops.

require_once or require is a suitable method in cases where a database connection file is involved and helps alleviate the possibility of multiple instances of the same file being included several times.

How can you tell if a number is even or odd without using any condition or loop?



echo "Your number is: ".$arr[$check%2];

What is the difference between session and quickies?

Session stores the value on the server and quickies stores the value on the user’s browser.

What does the follow code echo?

$a = "PHP";
$a = $a + 1;
echo $a;

The number 1.

$str = 'drinking giving jogging 喝 喝 passing 制图 giving 跑步 吃';  // Example input 

Highlight all the Chinese characters in red and return the string.

$str = 'drinking giving jogging 喝 喝 passing 制图 giving 跑步 吃';
$string = explode(' ', $str);
$chi = array_filter( explode('_', preg_replace(array('/[^\p{Han}?]/u', '/(\s)+/'), array('_', '$1'), $str)));
$value = array ();
foreach ($string as $s) {
           if (in_array($s, $chi)) {
               $value[] = ''.$s.'';
           } else {
               $value[] = $s;

return (implode(' ', $value));

Write a sample of code showing the nested ternary conditional operator in PHP.

$number_class = $number == 0 ? 'blue' : ($number > 0 ? 'green' : 'red');

Here we are assigning different strings to the number_class variable based on a numeric value ($number).

What are Traits?

Traits are a mechanism that provides some of the reuse advantages of multiple inheritance in languages like PHP where multiple inheritance is not supported. Traits enable developers to reuse combinations of methods from different class hierarchies.


10 PHP interview questions and answers – (upwork)

  • Q

    Do you have any experience with LAMP?


    LAMP is an open-source solution stack that contains everything you need to create a complete, stand-alone platform for your web application—Linux, Apache, MySQL, and PHP. This question is a good opportunity for your prospective programmer to detail any experience they have taking a web project from conception to completion.


  • Q

    What are PHP sessions and how do they work?


    What you’re really asking is whether they know how to use session_start(). It either creates or resumes a session based on an identifier that is sent to the server via a GET or POST request or a cookie. The most common use case scenario on the web is when a website won’t let you comment or post without first prompting a login. How does it know whether you’re logged in? One way would be to place a cookie in the user’s browser; on every request the cookie is sent server-side, where PHP can be used to determine which information is sent back and displayed to the client. While session_start() saves session data in files by default, it is also possible to store sessions directly in the database.


  • Q

    How are classes loaded in PHP?


    This is a good way to gauge a developer’s understanding of autoloading. Whenever a class is instantiated, a function is triggered. You can register a function with the spl_autoload que detailed below, courtesy of php.net:

    bool spl_autoload_register ([ callable $autoload_function [, bool$throw = true [, bool $prepend = false ]]] )


  • Q

    What are some of your favorite PHP design patterns?


    Design patterns are basically best-practice templates that programmers can use to consistently write well-designed code more quickly. Here are a couple examples along with their use cases:

    The Singleton: Used when you only want to allow access to one instance of a particular class.

    The Factory: The factory pattern makes it
    easier to create multiple classes without repeating code. To change,
    rename, or replace the class, simply modify the factory.


  • Q

    Determine the value of $pear after executing the code below. What will strlen($pear) return? Explain your answer.


    $pear = ‘PEAR ’;
    $pear[12] = ‘PHP Extension and Application Repository’;

    This question reveals a few interesting things about the way PHP interprets code. The value of $pear in the code above will be the string “PEAR P” or the string “PEAR ” followed by seven spaces, followed by “P,” which is the first character in the string “PHP Extension and Application Repository.” The value returned by strlen($pear) will thus be 13. Since an element of a string can only consist of one character, and the count of elements within a string starts with 0, $pear[12] sets the 13th character of the string to the letter “P.” Interestingly enough, we chose to set the 13th value of a string that only has five characters. While other language interpreters might have thrown an out-of-bounds index error, PHP is more forgiving and fills in the blanks with empty spaces.


  • Q

    Determine the output of the code below. Explain your answer.


    var_dump(42 == 042);
    var_dump(‘042’ == 42);
    var_dump(‘042’ === 42);

    This question quizzes the coder on how the PHP interpreter handles numbers and strings.
    i. var_dump(42 == 042); will output bool(false) because the PHP interpreter treats leading zeroes as octals. 042 is 32 in decimal, which does not equal 42.
    ii. var_dump(‘042’ == 42); will output bool(true) because PHP interpreter will coerce the string into an integer but ignore the leading zero. 42 is equal to 42.
    iii. var_dump(‘042’ === 42); will output bool(false) because the === operator performs a stricter comparison and will not coerce the integer into a string.


  • Q

    Explain why the two code snippets below will evaluate to the same output.


    $x = true or false; $x = true and false;
    var_dump($x); var_dump($x);

    This question is designed to test a coder’s knowledge of the order of operations. The first block of code is more straightforward, evaluating to true because of the “or” operator. The second code block, however, would typically be expected to evaluate to false. Instead, it evaluates to true because of how the PHP interpreter handles the order of operations: “=” is prioritized leading to the following equivalent code snippet:

    $x = true; // sets $x equal to true
    True and false; // evaluates to false, but does not affect anything


  • Q

    If $x = 100 + “25%” + “$40” what is the value of $x and why?


    The answer is $x = 125, because PHP uses automatic type conversion in deciding how to treat variables and values depending on how they are used. When PHP encounters a string during an arithmetic operation, it will interpret any numerical value and ignore the remainder in order from left to right. Strings that don’t start with numbers are evaluated to zero.

    $x = 100 + 25 + 0 = 125.


  • Q

    Explain the difference between classes and interfaces.


    In layman’s terms, an interface is a class without all the business logic. In an interface, all methods must be public and multiple inheritance is supported. However, all methods must be defined within the class that implements them. Abstract classes, on the other hand, can be declared with modifiers like public or internal, and can define properties or variables. Abstract classes do not support multiple inheritance and can only be extended by one abstract class.


  • Q

    Predict the output of the code below. Explain your answer.


    $x = 2
    $y = 4
    $z = 6
    if($z > $y > $x) {
        echo “true”;
        echo “false”;

    At first glance, one would expect the boolean to evaluate to “true” because 6 > 4 > 2. However, the correct answer is “false” because PHP will first evaluate $z > $y, which returns a boolean value of 1 or true. It is this boolean value of 1 that is compared to the next integer in the chain, bool(1) > $z, which will result in NULL and echo “false.”