Thank you for taking the time to write to us, but this is not
a bug. Please double-check the documentation available at
http://www.php.net/manual/ and the instructions on how to report
a bug at http://bugs.php.net/how-to-report.php

No, array_keys() isn't doing that. Numeric strings used as an array key are converted to integer when the element is created. This can be seen by removing array_keys() from your code:

<?php
$a = array('1'=>'abc','a'=>'1');
var_dump($a);
?>

And you will see:

array(2) {
  [1]=>
  string(3) "abc"
  ["a"]=>
  string(1) "1"
}

(Notice no quotes around the integer key.) And NOT:

array(2) {
  ["1"]=>
  string(3) "abc"
  ["a"]=>
  string(1) "1"
}