PHP mail() function is disabled through Apache.

Symptoms

Despite having no function disabled and the setting Prevent "nobody" from sending mail disabled in the WHM Tweak settings, the mail is still not being sent from PHP when executed from Apache:

[root@cptechs ~]cPs# php -i | grep disable_function
disable_functions => no value => no value

Usually, there are no errors logged when this issue occurs.

Description 

This is caused by utilizing the mod_mpm_itk Apache module. As mentioned in the following documentation:

The MPM ITK Apache module implements restrictions on the use of the setuid() function and the setgid() function. As a result, scripts that depend on these functions may encounter problems. This includes scripts that use the mail() function, the shell_exec function, or the sudo command.

- https://docs.cpanel.net/ea4/apache/apache-module-mpm-itk/#setuid-and-setgid-restrictions.

Workaround

Choose a different MPM: Which MPM should I choose?