Warning: Failed opening '...' for inclusion (include_path='...') in Unknown on line 0

I'm getting this warning message when I try to run any php code. Warning: Failed opening '...' for inclusion (include_path='...') in Unknown on line 0 I have had a look on bugs.php.net and yes I have checked that my paths are set correctly in php.ini so you can cross that off the list. I have tried running apache without any virtual hosts or mod_perl, I have even chopped the include_path in php.ini back to just '.' but with no luck. If I run php code with: <? phpinfo(); ?> It runs fine and reports everything as normal but <? echo "foo"; ?> will report the warning. I'm using: debian (currently 2.2.21) apache 1.3.26 php4 4.2.2 mod_perl 1.26 and all but the kernel where installed as packages using apt-get (yes the packages are in unstable but I have never had a problem before). The box (that I only just inherited and have never run apache/php on before) is running on is a p1-166, 8MB RAM, 100MB Swap on a 10GB Drive so I'm wondering if the fact my machine is low on resources is causing php to die.