Description:
------------
I was trying to associate values equals 1 for all the characters of a simple word with repeated characters with the function array_fill_keys.
For any reason I found that some keys were erased for any reason after using it like the example bellow.

Test script:
---------------
$word = 'metheor';
$arr = [];
echo strlen($word); //It shows the lenght of the word
for ($i = 0; $i < strlen($word); $i++){
  $arr[] = $word[$i];
}
print_r($arr); //It shows the full word
print_r(array_fill_keys($arr, 0)); //It shows the word filled with 0's but without repeated characters

Expected result:
----------------
array([m]=>1, [e]=>1, [t]=>1, [h]=>1, [e]=>1, [o]=>1, [r]=>1)

Actual result:
--------------
array([m]=>1, [e]=>1, [t]=>1, [h]=>1, [o]=>1, [r]=>1)