Use isset() instead of in_array() over array_keys() for key lookups

[mukesh27]

Several places in core test whether an array has a given key by building the full key list and scanning it linearly:

in_array( $key, array_keys( $array ), true )

This allocates a complete copy of every key and walks it — O(n) time and O(n) memory — where isset( $array[ $key ] ) does a single hash lookup in constant time with no allocation.

Trac ticket: https://core.trac.wordpress.org/ticket/65842

## Use of AI Tools

AI assistance: Yes
Tool(s): Claude
Model(s): Opus 5
Used for: Initial code skeleton and test suggestions; final implementation and tests were reviewed and edited by me.

[siliconforks]

Note that in some cases isset might not give the same result as in_array. For example, if the value of the array element is null, isset will return false.

<?php

$array = [
        'foo' => 1,
        'bar' => null,
        'baz' => 3,
];

$key = 'bar';

if ( in_array( $key, array_keys( $array ), true ) ) {
        echo 'in_array: true';
} else {
        echo 'in_array: false';
}
echo "\n";

if ( isset( $array[ $key ] ) ) {
        echo 'isset: true';
} else {
        echo 'isset: false';
}
echo "\n";

Running the above code gives:

in_array: true
isset: false

If you are certain that $array will never contain null values, then this is not an issue; but if you are not certain of this, it might be safer to use array_key_exists instead. (See the ​second example in the documentation for array_key_exists.)