Tips About PHP Memory Limit
1) Your WordPress memory limit cannot exceed your PHP memory limit. The php memory limit can be set, for example, editing your php.ini file.
For details about it, talk with your hosting company.
2) PHP memory_limit is per-script. For example, although PHP’s memory limit may be set high to 1GB, that does not mean that scripts will pile up to use that 1GB. This helps prevent poorly written scripts for eating up all available memory on a server… Then, I guess 256M is a good number for PHP ini and WP memory limit.
3) Your server need be free memory bigger than PHP Memory limit.
It’s important to understand that increasing the amount of memory each PHP process can use decreases the number of concurrent processes that can run.
This setting increases PHP Memory only for WordPress, not other applications
4) Administration tasks require much memory than usual operation. When in the administration area, the memory can be increased or decreased from the WP_MEMORY_LIMIT by defining WP_MAX_MEMORY_LIMIT.
define( ‘WP_MAX_MEMORY_LIMIT’, ‘256M’ );
Note: this has to be put before wp-settings.php inclusion.
Your WordPress Memory Limit Cannot Exceed Your WordPress Maximum Memory Limit.