I’m using PHP for an email-based order form using mail(). Because PHP is server-side, does this mean that there are no circumstances that PHP wouldn’t be able to communicate with the computer?
Is there a “disable PHP” option in any browsers like there is with JavaScript? Can I count on every browser with internet access being able to (or at least “doing it’s best to”) communicate with the PHP server?
Yes, PHP will work in all browsers.
PHP has nothing to do with browsers, its a server side language. PHP code is executed on the server, generating HTML which is then sent to the client. The client would receive the results of running that script.
check this link What is PHP?