You can specify the php version of each website separately by updating the .htaccess file.
To switch to PHP 4.4:
AddHandler application/x-httpd-php4 .php
To switch to PHP 5.0:
AddHandler application/x-httpd-php5 .php
To switch to PHP 5.1:
AddHandler application/x-httpd-php51 .php
To switch to PHP 5.2:
AddHandler application/x-httpd-php52 .php
To switch to PHP 5.3:
AddHandler application/x-httpd-php53 .php
To switch to PHP 5.4:
AddHandler application/x-httpd-php54 .php
To switch to PHP 5.5:
AddHandler application/x-httpd-php55 .php
To switch to PHP 5.6:
AddHandler application/x-httpd-php56 .php
To switch to PHP 7:
AddHandler application/x-httpd-php7 .php
To switch to PHP 7.1:
AddHandler application/x-httpd-php71 .php