deny access to a php file.

is there a script to deny access to a file from other sites using it? or something like if its not a certain domain, some other content shows up instead of the original file?

You can't deny access to a webpage as such, but if it's some content that is referred to in the page like an image, you can use the HTTP referrer to detect and block access from other domains.

its a php file that im trying to block access to. so theres no way to give false information for people accessing it outside the domain?

Did you read this? You can detect and block access from other domains.
Also you can use a .htaccess file if it is in a separate folder from the rest of your site and deny all outside domains.
Rider

You can try create .htaccess :

RewriteCond %{HTTP_HOST} ^domain_to_be_redirected\.com

put this on folder where php file you want to protect