First, understand that you have three languages working together:
PHP: It only runs by the server and responds to requests like clicking on a link (GET) or submitting a form (POST).
HTML & JavaScript: It only runs in someone's browser (excluding NodeJS).
I'm assuming your file looks something like:
<!DOCTYPE HTML>
<html>
<?php
function runMyFunction() {
echo 'I just ran a php function';
}
if (isset($_GET['hello'])) {
runMyFunction();
}
?>
Hello there!
<a href='index.php?hello=true'>Run PHP Function</a>
</html>
Because PHP only responds to requests (GET, POST, PUT, PATCH, and DELETE via $_REQUEST), this is how you have to run a PHP function even though they're in the same file. This gives you a level of security, "Should I run this script for this user or not?".
If you don't want to refresh the page, you can make a request to PHP without refreshing via a method called Asynchronous JavaScript and XML (AJAX).
That is something you can look up on YouTube though. Just search "jquery ajax"
This is the easiest possible way. If form is posted via post, do php function. Note that if you want to perform function asynchronously (without the need to reload the page), then you'll need AJAX.
Here´s an alternative with AJAX but no jQuery, just regular JavaScript:
Add this to first/main php page, where you want to call the action from, but change it from a potential a tag (hyperlink) to a button element, so it does not get clicked by any bots or malicious apps (or whatever).
<head>
<script>
// function invoking ajax with pure javascript, no jquery required.
function myFunction(value_myfunction) {
var xmlhttp = new XMLHttpRequest();
xmlhttp.onreadystatechange = function() {
if (this.readyState == 4 && this.status == 200) {
document.getElementById("results").innerHTML += this.responseText;
// note '+=', adds result to the existing paragraph, remove the '+' to replace.
}
};
xmlhttp.open("GET", "ajax-php-page.php?sendValue=" + value_myfunction, true);
xmlhttp.send();
}
</script>
</head>
<body>
<?php $sendingValue = "thevalue"; // value to send to ajax php page. ?>
<!-- using button instead of hyperlink (a) -->
<button type="button" onclick="value_myfunction('<?php echo $sendingValue; ?>');">Click to send value</button>
<h4>Responses from ajax-php-page.php:</h4>
<p id="results"></p> <!-- the ajax javascript enters returned GET values here -->
</body>
When the button is clicked, onclick uses the the head´s javascript function to send $sendingValue via ajax to another php-page, like many examples before this one. The other page, ajax-php-page.php, checks for the GET value and returns with print_r: