Description:
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Currently whenever an email is sent through the mail() function it is sent by an invocation of a sendmail-compatible executable. However, there are scenario's in which PHP has been locked down, preventing it from starting any executables.

It would be nice if it would be possible to allow alternative ways of transporting an email message than only through sendmail, e.g. through a PHP Extension.

This approach is akin to how sessions have a storage handler. PHP by default provides the 'file' and 'user' handlers. But the memcached extension provides a storage mechanism of its own which is then registered through php_session_register_module().

This would only implement an extra hook to allow for overriding the default sendmail mail transport without bringing extra functionality to the php user land.

Test script:
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I'd like to propose this patch to implement this functionality: https://github.com/php/php-src/pull/1778