You need to increase your PHP memory limits.
This is the recommended approach if you have access to the server's php.ini. This will not be possible in some shared hosting environments, though your host may be able to adjust it for you. Note that this change will affect all websites and PHP scripts on the server.
Locate the php.ini file used by your web server.
Edit the memory_limit parameter in the php.ini file (usually in a section called Resource Limits). Make sure you use M to specify the number of megabytes (not MB).
memory_limit = 256M ; Maximum amount of memory a script may consume (256MB)
If there is no section already for this, place the above line at the end of the file.