How to check a key exists in an array in PHP ?

We have given an array arr and a Key key, the task is to check if a key exists in an array or not in PHP.

Examples:

Input : arr = ["Geek1", "Geek2", "1", "2","3"] 
        key = "2"
Output : Found the Key

Input : arr = ["Geek1", "Geek2", "1", "2","3"] 
        key = 9
Output : Key not Found

The problem can be solved using PHP inbuilt function for checking key exists in a given array. The in-built function used for the given problem are:

Method 1: Using array_key_exists() Method:

The array_key_exists() function checks whether a specific key or index is present inside an array or not.

Syntax:

boolean array_key_exists( $index, $array )

Example:

<?php
// PHP program to check if a key 
// exists in an array or not

$array = array(
      'names' => array("Geek1", "Geek2", "Geek3"), 
    'rank' => array('1', '2', '3')
); 

// Use of array_key_exists() function
if(array_key_exists("rank", $array)) {
    echo "Found the Key"; 
} 
else{ 
    echo "Key not Found"; 
} 

?>

Found the Key

Method 2: Using isset() Method:

The isset() function checks whether a specific key or index is present inside an array or not.

Syntax:

bool isset( mixed $var, mixed $... )

<?php
// PHP program to check if a key 
// exists in an array or not

$array = array(
      'names' => array("Geek1", "Geek2", "Geek3"), 
    'rank' => array('1', '2', '3')
); 

// Use of array_key_exists() function
if(isset($array["rank"])){
    echo "Found the Key"; 
} 
else{ 
    echo "Key not Found"; 
} 

?> 

Found the Key 

Method 3: Using the array_search function:

The array_search function is used to search for a given value in an array and return the corresponding key if the value is found. If the value is not found, it returns false.

Syntax:

bool = array_search($search_value, $array);

<?php
    $array = array('name' => 'geek', 'age' => '22');

if (array_search('geek', $array) !== false) {
    echo 'Found the key';
} else {
    echo 'Key not found';
}

?>

Found the key

Method 4: Using array_keys

Using array_keys to check if a key exists involves getting all the keys of the array with array_keys and then using in_array to determine if the specific key is present

Example

<?php
$array = ['name' => 'John', 'age' => 30, 'city' => 'New York'];
$key = 'age';

if (in_array($key, array_keys($array))) {
    echo "Key '$key' exists in the array.";
} else {
    echo "Key '$key' does not exist in the array.";
}
?>

Key 'age' exists in the array.

Using array_key_exists() with array_flip()

This approach involves flipping the array's keys and values using array_flip(), then using array_key_exists() to check if the key exists in the flipped array.

Example:

<?php
function keyExistsUsingArrayFlip($array, $key) {
    $flippedArray = array_flip($array);
    return array_key_exists($key, $flippedArray);
}

// Example usage:
$arr = ["Geek1", "Geek2", "1", "2", "3"];
$key = "2";
if (keyExistsUsingArrayFlip($arr, $key)) {
    echo "Found the Key";
} else {
    echo "Key not Found";
}

?>

Found the Key

Using in_array Function Directly

The in_array function checks if a specific value exists in an array, returning true if found and false otherwise. It's used for value existence checks, not for keys.

Example

<?php

$arr = ["Geek1", "Geek2", "1", "2", "3"];
$key = "2";

if (in_array($key, $arr)) {
    echo "Found the Key";
} else {
    echo "Key not Found";
}

?>

Found the Key