str_replace not working?

[Jmacp]

Must be missing something here but str_replace isnt working for me, take this simple example,

<?php

$a = "http://somesite.com/index.cfm?&user=24706091";
$b = str_replace("&","&amp;",$a);
echo $b;

?>

Result is still the original url whereas it should be,

http://somesite.com/index.cfm?&user=24706091