To change the PHP version for your website, please do the following:
Login to your 123-reg control panel.
Scroll down to the web hosting section.
Select the relevant domain name from the drop down box and click on the Manage button.
Click on the Switch PHP Version option in the Website Help & Diagnostics section.
If you need to make the .php files on your website execute as PHP5.2 instead of PHP5.3 press the Switch to PHP 5.2 button.
Alternatively, if you would like to enable PHP 5.5, you will need to add the below to a root.htacces file:
AddType application/x-httpd-php55 .php
You can also label your files with the .php55 extension.