Array search functions in PHP - Cheat Sheet & Quick Revision

Recall & Review

beginner

What does the PHP function in_array() do?

It checks if a value exists in an array and returns true if found, otherwise false.

beginner

How does array_search() differ from in_array()?

array_search() returns the key of the found value in the array, or false if not found, while in_array() returns only true or false.

beginner

What is the return value of array_search() if the searched value is not found?

It returns false to indicate the value is not present in the array.

intermediate

Explain the use of the third parameter in in_array() and array_search().

The third parameter is a boolean for strict checking. If true, the functions check both value and type (===). If false or omitted, only value is checked (==).

intermediate

What will array_search('5', [1, 2, 3, 5]) return with strict mode off and on?

With strict mode off (default), it returns the key 3 because '5' loosely equals 5.
With strict mode on, it returns false because string '5' is not identical to integer 5.

Which PHP function returns the key of a value in an array?

A array_search()

B in_array()

C array_key_exists()

D array_keys()

array_search() returns the key of the found value. in_array() returns true/false only.

What does in_array('10', [10, 20, 30]) return by default?

A 10

B false

C true

D null

By default, in_array() does loose comparison, so string '10' matches integer 10.

If array_search() does not find the value, what does it return?

A false

B -1

C null

D 0

array_search() returns false when the value is not found.

Which parameter controls strict type checking in in_array()?

A First parameter

B No parameter controls this

C Second parameter

D Third parameter

The third parameter is a boolean that enables strict type checking.

What will array_search(5, ['5'], true) return?

A 0

B false

C 1

D 5

With strict mode on, integer 5 is not identical to string '5', so it returns false.

Describe how to check if a value exists in an array and get its key in PHP.

Think about two functions: one for yes/no, one for key.

Explain the difference between loose and strict checking in PHP array search functions.

Consider how '5' and 5 compare with and without strict mode.

Practice

1. Which PHP function returns the key of a value in an array if found, or false if not found?

easy

A. in_array

B. array_search

C. array_key_exists

D. array_keys

Solution

1. Step 1: Understand function purpose

   array_search looks for a value and returns its key if found.

2. Step 2: Compare with other functions

   in_array returns true/false, not key; array_key_exists checks keys; array_keys returns all keys.

3. Final Answer:

   array_search -> Option B

4. Quick Check:

   Key returned by array_search = array_search [OK]

Hint: array_search returns key, in_array returns true/false [OK]

Common Mistakes:

• Confusing array_search with in_array
• Thinking array_key_exists returns value
• Assuming array_keys searches values

2. Which of the following is the correct syntax to check if the value 5 exists in array $arr using in_array?

easy

A. in_array(5, $arr);

B. in_array($arr, 5);

C. in_array($arr => 5);

D. in_array(5 => $arr);

Solution

1. Step 1: Recall in_array syntax

   The correct syntax is in_array(value, array).

2. Step 2: Match syntax with options

   in_array(5, $arr); matches in_array(5, $arr); which is correct.

3. Final Answer:

   in_array(5, $arr); -> Option A

4. Quick Check:

   in_array(value, array) = in_array(5, $arr) [OK]

Hint: Remember in_array(value, array) order [OK]

Common Mistakes:

• Swapping parameters order
• Using arrow syntax instead of comma
• Passing array as first argument

3. What is the output of the following PHP code?

$arr = ['apple', 'banana', 'cherry'];
$key = array_search('banana', $arr);
echo $key;

medium

A. 0

B. banana

C. false

D. 1

Solution

1. Step 1: Understand array_search behavior

   It returns the key of the searched value if found.

2. Step 2: Check array keys and searched value

   Array keys are 0=>'apple', 1=>'banana', 2=>'cherry'. Searching 'banana' returns key 1.

3. Final Answer:

   1 -> Option D

4. Quick Check:

   array_search('banana', $arr) = 1 [OK]

Hint: array_search returns index/key of found value [OK]

Common Mistakes:

• Expecting value instead of key
• Confusing false with 0
• Assuming it returns boolean

4. Identify the error in the following PHP code snippet:

$arr = ['x' => 10, 'y' => 20];
if (array_search(30, $arr)) {
    echo 'Found';
} else {
    echo 'Not Found';
}

medium

A. array_search returns false but condition treats 0 as false

B. array_search returns key but 30 is not in array, so condition fails correctly

C. array_search should be replaced with in_array for this check

D. No error, code works as expected

Solution

1. Step 1: Analyze array_search return and condition

   array_search returns key or false. If key is 0, condition fails because 0 is falsey.

2. Step 2: Check searched value presence

   30 is not in array, so array_search returns false, condition is false, prints 'Not Found'. But if value was at key 0, condition would fail incorrectly.

3. Step 3: Suggest correct function for boolean check

   Use in_array which returns true/false directly for presence check.

4. Final Answer:

   array_search should be replaced with in_array for this check -> Option C

5. Quick Check:

   Use in_array for boolean presence check [OK]

Hint: Use in_array for true/false presence checks [OK]

Common Mistakes:

• Using array_search in if without strict check
• Ignoring 0 key treated as false
• Not using === to check false

5. Given the array $data = ['a' => 0, 'b' => false, 'c' => null, 'd' => '0'];, which function call correctly checks if the value 0 exists in the array considering type?

hard

A. in_array(0, $data, true);

B. array_search(0, $data);

C. in_array(0, $data);

D. array_search('0', $data, true);

Solution

1. Step 1: Understand strict checking

   Using third parameter true enforces type checking in in_array and array_search.

2. Step 2: Analyze array values and types

   Values are 0 (int), false (bool), null, and '0' (string). Only strict check distinguishes 0 (int) from others.

3. Step 3: Choose correct function and parameters

   in_array(0, $data, true) returns true only if exact int 0 found. array_search without strict may confuse types.

4. Final Answer:

   in_array(0, $data, true); -> Option A

5. Quick Check:

   Use in_array with strict=true for exact type match [OK]

Hint: Use in_array with strict=true to check exact value and type [OK]

Common Mistakes:

• Ignoring strict parameter causing false positives
• Using array_search without strict for type-sensitive check
• Confusing '0' string with 0 integer