It seems like every once in a while I run into php memory problems. It all comes down to the php.ini file. If you just need to increase the file upload size for the WordPress backend you can modify the php5.ini file in the root folder where WordPress is installed, however if you are downloading a large file from the server you need to modify the servers base php.ini file.
The weirdest problem I ran into was after trying to force download a large file it would throw a 500 error and say it couldn’t find the file even though I knew that the file did indeed exist. When checking the apache error log it showed that it was actually a memory allocation error and that is what lead me to change the php.ini file.