Shouldn't be any harder than adding a line like this to the top of each PHP script ....
#!/usr/bin/php
Then, you just need to set the execute bit on each of the scripts. If you are on *nix, you can do a command like this for the whole directory (and subdirectories) ...
chmod 700 -fR my_directory
or ...
chmod u+rwx -fR my_directory
This will give the file owners read, write and execute permissions on all scripts in the dir. You may need to set different perms depending on the user that actually calls the script.