(PHP 4 >= 4.2.0, PHP 5, PHP 7)
array_change_key_case — Changes the case of all keys in an array
$array
   [, int $case = CASE_LOWER
  ] )
   Returns an array with all keys from array lowercased
   or uppercased. Numbered indices are left as is.
  
arrayThe array to work on
case
       Either CASE_UPPER or
       CASE_LOWER (default)
      
   Returns an array with its keys lower or uppercased, or FALSE if
   array is not an array.
  
   Throws E_WARNING if array is
   not an array.
  
Example #1 array_change_key_case() example
<?php
$input_array = array("FirSt" => 1, "SecOnd" => 4);
print_r(array_change_key_case($input_array, CASE_UPPER));
?>
The above example will output:
Array
(
    [FIRST] => 1
    [SECOND] => 4
)
Note:
If an array has indices that will be the same once run through this function (e.g. "keY" and "kEY"), the value that is later in the array will override other indices.