Allowed memory size of X bytes exhausted

I had same issue. I found the answer:

ini_set('memory_limit', '-1');

Note: It will take unlimited memory usage of server.

Update: Use this carefully as this might slow down your system if the PHP script starts using an excessive amount of memory, causing a lot of swap space usage. You can use this if you know program will not take much memory and also you don't know how much to set it right now. But you will eventually find it how much memory you require for that program.

You should always memory limit as some value as answered by @şarkı dinle.

ini_set('memory_limit', '512M');

Giving unlimited memory is bad practice, rather we should give some max limit that we can bear and then optimise our code or add some RAMs.

ini_set('memory_limit', '128M'); 

or

php.ini  =>  memory_limit = 128M

or

php_value memory_limit 128M

PHP's config can be set in multiple places:

1. master system php.ini (usually in /etc somewhere)
2. somewhere in Apache's configuration (httpd.conf or a per-site .conf file, via php_value)
3. CLI & CGI can have a different php.ini (use the command php -i | grep memory_limit to check the CLI conf)
4. local .htaccess files (also php_value)
5. in-script (via ini_set())

In PHPinfo's output, the "Master" value is the compiled-in default value, and the "Local" value is what's actually in effect. It can be either unchanged from the default, or overridden in any of the above locations.

Also note that PHP generally has different .ini files for command-line and webserver-based operation. Checking phpinfo() from the command line will report different values than if you'd run it in a web-based script.