Php Limits

php

ThreadStackSize 16*1024*1024
Check your Apache config (e.g., httpd.conf). There's likely an RLimitMEM directive limiting the memory allow for children processes handling requests.

https://stackoverflow.com/questions/4399138/upper-memory-limit-for-php-apache
https://www.digitalocean.com/community/tutorials/how-to-optimize-apache-web-server-performance
https://serverfault.com/questions/112361/limit-apache-2-memory-usage
https://aws.amazon.com/premiumsupport/knowledge-center/ec2-apache-memory-tuning/

// in php.ini
memory_limit = 32M
memory_limit = 3G

set_time_limit(0);

// in php code
ini_set("memory_limit", "-1");
ini_set('memory_limit', '128M');

// in .htaccess
php_value memory_limit 64M

<IfModule prefork.c>
        MaxRequestsPerChild  1
</IfModule>