Increase PHP Memory
If you are receiving error messages telling you that your “Allowed memory size of xxx bytes exhausted,” this setting may help resolve the issue. As of WordPress version 2.5, the WP_MEMORY_LIMIT definition enables you to specify the maximum amount of memory that may be used by PHP. Here are some examples:
By default, WordPress will automatically attempt to increase PHP memory up to 32MB, so this setting is only needed for values higher than 32MB. Note that some web hosts disable your ability to increase PHP memory, so you may need to ask (or beg) for them to do it.