You can set a custom PHP version per domain or directory by creating a .htaccess file containing the following text or by placing it at the top of an existing .htaccess file (choose the appropriate version):
# PHP 5.3
<FilesMatch "\.(php4|php5|php3|php2|php|phtml)$">
SetHandler application/x-httpd-alt-php53___lsphp
</FilesMatch>
# PHP 5.4
<FilesMatch "\.(php4|php5|php3|php2|php|phtml)$">
SetHandler application/x-httpd-alt-php54___lsphp
</FilesMatch>
# PHP 5.5
<FilesMatch "\.(php4|php5|php3|php2|php|phtml)$">
SetHandler application/x-httpd-alt-php55___lsphp
</FilesMatch>
# PHP 5.6
<FilesMatch "\.(php4|php5|php3|php2|php|phtml)$">
SetHandler application/x-httpd-alt-php56___lsphp
</FilesMatch>
# PHP 7.0
<FilesMatch "\.(php4|php5|php3|php2|php|phtml)$">
SetHandler application/x-httpd-alt-php70___lsphp
</FilesMatch>
# PHP 7.1
<FilesMatch "\.(php4|php5|php3|php2|php|phtml)$">
SetHandler application/x-httpd-alt-php71___lsphp
</FilesMatch>
# PHP 7.2
<FilesMatch "\.(php4|php5|php3|php2|php|phtml)$">
SetHandler application/x-httpd-alt-php72___lsphp
</FilesMatch>
# PHP 7.3
<FilesMatch "\.(php4|php5|php3|php2|php|phtml)$">
SetHandler application/x-httpd-alt-php73___lsphp
</FilesMatch>
# PHP 7.4
<FilesMatch "\.(php4|php5|php3|php2|php|phtml)$">
SetHandler application/x-httpd-alt-php74___lsphp
</FilesMatch>
# PHP 8.0
<FilesMatch "\.(php4|php5|php3|php2|php|phtml)$">
SetHandler application/x-httpd-alt-php80___lsphp
</FilesMatch>
# PHP 8.1
<FilesMatch "\.(php4|php5|php3|php2|php|phtml)$">
SetHandler application/x-httpd-alt-php81___lsphp
</FilesMatch>
As with any .htaccess file it works recursively so will affect all directories below the one it's in, if you need anything below to use a different version you can place another .htaccess file with the appropriate version in it.