PHP's build system has an awesome feature: every one of the included extensions can also be built stand-alone. There is no difference between an external and a bundled library.
So to add a bundled library to the mac's PHP, you download the PHP sources (most of the time, not even the version has to match) and you extract them.
Then you cd into ext/<name of extension> and type phpize, followed by configure and make install.
Now you can enable the freshly built and correctly installed extension in your php.ini.
No need to replace the system PHP with something home-grown.
So to add a bundled library to the mac's PHP, you download the PHP sources (most of the time, not even the version has to match) and you extract them.
Then you cd into ext/<name of extension> and type phpize, followed by configure and make install.
Now you can enable the freshly built and correctly installed extension in your php.ini.
No need to replace the system PHP with something home-grown.