<?php
 
$array = array('a' => 1, 'd' => 2, 'c' => 3); //associative array
 
// rename $array['d'] as $array['b']
$array = replace_key_function($array, 'd', 'b');
 
var_export($array); // array('a' => 1, 'b' => 2, 'c' => 3); same order!
 
function replace_key_function($array, $key1, $key2)
{
    $keys = array_keys($array);
    $index = array_search($key1, $keys);
 
    if ($index !== false) {
        $keys[$index] = $key2;
        $array = array_combine($keys, $array);
    }
 
    return $array;
}

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