I am running:
Apache/1.3.12 (Unix) mod_perl/1.21 PHP/4.0RC1 mod_ssl/2.6.2 OpenSSL/0.9.5

The problem is that in_array matches string vars as integer vars, for example the following code will work incorrectly:

$a[0]="001";
$a[1]="003";
if (in_array("1", $a))
 {
 echo "As if<br>";
 }

This is a big problem since it will cause the following code to produce an error:

$HAObjects["0011"]=$Object;
$HAObjects["0131"]=$Object;

if (in_array("11", array_keys($HAObjects)))
 {
 $O=$HAObject["11"];
 $O->SomeFunction();
 }