Calling mcrypt_generic_init with an empty $key causes a segmentation fault in Apache. Obviously $key should not be empty, but errors will be made, so perhaps a parse error could be generated in this event rather than a crash.

Example Code
<?php
//Open encryption module
$td = mcrypt_module_open (MCRYPT_ARCFOUR, "", MCRYPT_MODE_STREAM, "");
srand ((double) microtime() * 1000000);
$iv = mcrypt_create_iv (mcrypt_enc_get_iv_size ($td), MCRYPT_RAND);

//Encrypt data
$data = "message";
mcrypt_generic_init($td, $key, $iv);//HERE IS THE PROBLEM
$encrypted_data = mcrypt_generic ($td, $data);
mcrypt_generic_end ($td);
?>