It is quite easy to crash the PHP interpreter with infinite recursion.  While this is a user error, the interpreter should not crash.  There should be some sort of internal checking to assure that too many function calls cannot cause a crash.

<?
    foo();
    function foo() { foo(); }
?>

[Tue Feb 12 07:09:45 2002] [notice] child pid 4079 exit signal Segmentation fault (11)