PHP Tricky Interview Questions and answers

Umar Farooque Khan
4 min readDec 19, 2023

Delve into the depths of PHP intricacies with this assortment of challenging interview questions and answers. This compilation covers a broad spectrum of PHP topics, evaluating your comprehension of arrays, strings, operator precedence, and loop behavior. Whether you’re demystifying the complexities of array conversions or deciphering unexpected results in logical operations, each question provides an opportunity to enhance your understanding and hone your PHP proficiency. Navigate through array transformations, grasp the subtleties of increment and decrement operations, and unravel the outcomes of loops. These questions are designed to not only challenge but also refine your skills in the intricate realm of PHP. Let’s embark on this exploration of PHP intricacies together.

Question 1:

What is the output of the following code?

$array = array(1 => "a", "1" => "b", 1.5 => "c", true => "d");

echo $array[1];

Output:

d

Explanation:

The code outputs “d” because PHP converts disparate keys like 1, “1”, 1.5, and true to the common type integer (1). Consequently, the last assignment to key 1, “d”, overwrites previous values. Echoing $array[1] retrieves the value associated with the integer key 1, yielding “d”. PHP implicitly converts keys to a common type, unifying them in this case to the integer 1, resulting in the final association with “d”.

Question 2:

What does the following PHP code output, and why?

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

Output:

bool(true)

Explanation:

The output is bool(true). Despite the apparent contradiction, the expression $x = true and false; is parsed as $x = (true and false);. The assignment operation has higher precedence than the logical AND. Consequently, the result of (true and false) (evaluating to false) is assigned to $x, leading to the unexpected output when var_dump($x) is called: bool(true). Understanding operator precedence in PHP is crucial for interpreting such seemingly contradictory outcomes.

Question 3:

What does the following PHP code output, and why?

$array = array(1, 2, 3, 4, 5);
$array = array_map(function($value) {
return $value * 2;
}, $array);

echo implode(", ", $array);

Output:

2, 4, 6, 8, 10

Explanation:

The code outputs “2, 4, 6, 8, 10”. The array_map function applies the provided callback (multiplying each element by 2) to all elements of the array. Therefore, the resulting array contains each original value doubled. The implode function then joins the elements into a string separated by ", ", resulting in the final output. This example illustrates the efficient use of array_map for concise array transformations and demonstrates the power of array manipulation in PHP.

Question 4:

What does the following PHP code output, and why?

<?php
$number = 3;
echo $number;
echo $number++ + $number++;
echo $number;
echo $number-- - $number--;
echo $number;
?>

Output:

37513

Explanation:

The code illustrates the concept of per-increment, post-increment, per-decrement, and post-decrement operations in PHP. It initializes a variable to 3, manipulates its value with increment and decrement operators in different expressions, and prints the results. This highlights the importance of understanding the order of operations, showcasing how it influences the final output of the code.

Question 5:

What does the following PHP code output, and why?

$array = [1, 2, 3, 4, 5];
unset($array[2]);
$result = array_values($array);

echo implode(", ", $result);

Output:

1, 2, 4, 5

Explanation:

The code removes the element at index 2 from the array [1, 2, 3, 4, 5], re indexes the array using array_values, and then prints the result using implode. The output is "1, 2, 4, 5," demonstrating the removal and re indexing of array elements.

Question 6:

What does the following PHP code output, and why?

$person = 'John ';
$person[10] = 'Smith';
echo $person;

Output:

John      S

Explanation:

The code initializes a string $person with 'John ' and replaces the character at index 10 with 'Smith'. The output is 'John S' due to the replacement, which includes additional spaces.

Question 7:

What does the following PHP code output, and why?

$array = ['a' => 'apple', 'b' => 'banana', 'c' => 'cherry'];
unset($array['b']);
$result = array_keys($array);

echo implode(", ", $result);

Output:

a, c

Explanation:

Using unset on 'b' removes it from the array. The keys remaining are 'a' and 'c'. array_keys extracts these keys, and implode combines them into the output "a,

Question 8:

What does the following PHP code output, and why?

$array = [1, 2, 3, 4, 5];
foreach ($array as &$value) {
$value *= 2;
}
unset($value);
$result = array_map(function($v) { return $v - 1; }, $array);
echo implode(", ", $result);

Output:

1, 3, 5, 7, 9

Explanation:

Question 9:

What does the following PHP code output, and why?

$count = 7;

while (--$count > 0) {
$count++;
echo $count;
echo "hello";
}

Output:

7hello7hello7hello7hello7hello
RangeError [ERR_CHILD_PROCESS_STDIO_MAXBUFFER]: stdout maxBuffer length exceeded

Explanation:

The code results in an infinite loop because the termination condition --$count > 0 is always true, leading to continuous printing. The repeated output exceeds the maximum buffer length for standard output, causing a RangeError. To fix, adjust the loop condition to eventually evaluate to false and prevent an infinite loop.

Conclusion:

In conclusion, these PHP interview questions have navigated through various facets of the language, testing your expertise in arrays, strings, logical operations, and loop mechanisms. By unraveling intricacies and decoding unexpected outcomes, you’ve had the opportunity to sharpen your PHP skills. As you explored the nuances of array transformations, delved into the subtleties of increment and decrement operations, and deciphered the results of loops, this journey aimed to challenge and refine your PHP proficiency. Armed with a deeper understanding of PHP intricacies, you’re better equipped to tackle the diverse challenges the language presents. Happy coding!

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Umar Farooque Khan

Experienced software developer with a passion for clean code and problem-solving. Full-stack expertise in web development. Lifelong learner and team player.