I might be wrong, But I don't think cPanel/WHM update the PHP verison on its own.
It's the server owner who updates the PHP version to a new one.
If you are using web hosting you should ask your web hoster not to update PHP for that server.
Webhosters that cares about customers usually update PHP step by step and leavs the old version of the PHP too, so you can choose playing with Apache Handlers which PHP version to use.
For instance you could set the files with ".php5" extension to be processed by PHP 5.3, while all the files with standard ".php" extension to be processed by the old version of PHP. In the .htaccess you can write:
#Makes PHP5.3 to process only .php5 extension file
AddHandler application/x-httpd-php5 .php5
#Makes PHP4 to process all .php file
AddHandler application/x-httpd-php4 .php
application/x-httpd-php5 has to be created by the webhoster that installe both PHP versions.
This is an example of a webhoster who cares www.qualityhostonline.com about their customers as explained above and they provide what I explained.
Unfortunately many cheap webhosters don't take the time to do it, so they update PHP all of a sudden, and the next day they send you an email saying: "Oh we updated PHP, enjoy!" and you might get all bunch of errors in your apps with hunderds of customer calling you screaming.