<?php

$mystring = "1";

$myarray = array();
$myarray[(string)$mystring] = "my content of a key which is supposed to be STRING!!!";

var_dump($myarray);

RESULT
------

array(1) {
  [1]=>
  string(53) "my content of a key which is supposed to be STRING!!!"
}

YES! the key is integer, FOR FUCK's SAKE, THANK YOU PHP!