Can be done through .htaccess.
Each .htaccess can have the lines:
AddHandler cust-php .php
Just replace with the path to the specific PHP binaries you wish to use. This will overwride any PHP settings defined within Apache (even the module). One .htaccess file per directory (which you want to change)
Here's how I do mine, note I have multiple versions set:
The .sphp and .phps is using the Apache module PHP Syntax parser.