Change PHP executable find order (and handle PHP_PATH options)

[Maxeee09]

Change PHP executable detection order to allow PHP_PATH environment variable usage and override PHP_BINARY detection behavior. Specifically when you need to use a different php.ini.

[tompizmor]

Hi,

I noticed that this commit will solve an issue on OS X 10.11 El Capitan. The exact issue is that El Capitan added a new “System Integrity Protection (SIP)” that reduces the attack surface for malware that relies on modifying system files.
In some cases could be necessary to run PHP wrappers and with the current detection order is impossible to specify the PHP binary to execute.

[1]
https://www.apple.com/osx/all-features/pdf/osx_elcapitan_core_technologies_overview.pdf
See page 16 “System Integrity Protection"

Tomas

[nicolas-grekas]

Thanks for this PR. Yet I invite you to submit it on the main symfony/symfony repository: this is where merges happen. Note that you should add test cases while doing so. Cheers.