The amount of memory that PHP uses is controlled by the php.ini file. The php.ini file is located in /usr/local/lib directory. In this file, if you want to change the alloted memory for script execution, you need to find a line that looks like the following:
memory_limit = 16M
This is change in megabytes, notice the M. Remember, that you ALWAYS have to specify M, and NOT MB, or you will cause the ini file not to be read properly, and have only 16 bytes if you have 16MB specified in that line.
Save the file and upload it back to the server. Ensure you make a backup copy before you upload it.