PHP - POST string containing >< causes error

PHP POST String containing >< causes 500 Internal Server Error

I am trying to figure out why on one of my servers (Linux - php 5.3.6) I am getting an error when I try to POST a string containing “><” but on another server (Linux - php 5.2.17) I am not getting an error.

I’ve tried these functions to sorta sanitize the data

addslashes()
htmlentities()
htmlspecialchars()
mysql_real_escape_string()
strip_tags()

All of them fail on the 5.3.6 server when the posted string contains “><”. Is there a function/setting I’m missing. This has throughly confused me.

This is the error I’m getting:
Forbidden
You don’t have permission to access post.php on this server.
Additionally, a 500 Internal Server Error error was encountered while trying to use an ErrorDocument to handle the request