How Can I Preserve Keys When Filtering an Array in PHP?

Preserving Keys in Array Filtering Using array_intersect_key and array_flip

In PHP, the array_filter() function can be used to remove elements from an array based on a custom callback function. However, the callback only receives the values of the array, not the keys.

Consider the scenario where you have an associative array ($my_array) and a list of allowed keys in an indexed array ($allowed). The goal is to filter $my_array and remove all keys that are not present in $allowed.

The most effective approach is to leverage the array_intersect_key and array_flip functions. The array_intersect_key function filters an array key-value pairs based on the keys present in another array. The array_flip function flips the keys and values of an array, effectively creating a mapping between values and keys.

By combining these functions, you can achieve the desired filtering:

$filtered_array = array_intersect_key($my_array, array_flip($allowed));

This code will create a new array ($filtered_array) that contains only the key-value pairs from $my_array where the keys are present in $allowed. The result will be identical to the desired output:

array(
  ["foo"] => 1
)

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