Description:
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I would like to recommend that a feature be added to PHP 5.3 or 6.0 that will allow the execution of an application using a different version of the runtime.

This could be done via the php.ini. For example:

[interpreters]
php44 = "c:/php4/php.exe"
php51 = "c:/php51/php-cgi.exe"
php5 = "c:/php52/php-cgi.exe"

[interpreter_mapping]
php44 = "c:/website/oldapps/addressbook/";
php5 = "c:/website/currentapps/crm/";

With the above, the current version of PHP would be the default interpreter. This means that if no mapping was found for the current path then the current interpreters will be used, otherwise system should pass control over to the selected interpreter. 

This technique might be a little slower when passing control from the current to an older interpreter but it gives the user greater flexibility and more time to work with both the new php releases and the older interpreters. Plus it allows applications designed for older versions of php to run side-by-side on same server with no special configuration changes to the server or file settings.