Running different versions per site.
WHY IT ARISES
Older applications require older versions, while new ones need current ones.
WHAT MAKES IT POSSIBLE
Running PHP as a separate process pool per version, rather than inside the web server.
WHAT THE SYSTEM PHP IS
The version installed for command line use, which is frequently not the one serving sites.
WHY THAT CAUSES CONFUSION
A script behaves differently on the command line than through the browser.
HOW TO SEE WHICH VERSION SERVES A SITE
A page reporting the configuration, or the control panel.
HOW TO RUN A SCRIPT WITH A SPECIFIC VERSION
Call that version's binary by its full path.
WHY THAT MATTERS FOR SCHEDULED TASKS
A cron job using plain php may use a version the application does not support.
WHAT TO ALWAYS DO IN CRON
Use the full path to the intended binary.
WHAT CONFIGURATION DIFFERS PER VERSION
Memory limits Execution time Upload sizes Extensions loaded
WHERE TO CHANGE THEM
The version-specific configuration, or the control panel.
WHAT TO CHECK AFTER CHANGING A VERSION
That the site loads That scheduled tasks still work That required extensions are present
WHAT COMMONLY BREAKS ON UPGRADE
Removed functions Stricter type handling
WHAT TO DO
Test on a copy first.
