Como executar PHP em JavaScript

// PHP is a Server Side Language, and JS is a Client Side Language, so the simple answer is  
// you can not "run php in javascript". You CAN, however, use PHP to render an html 
// page that includes js code on the server, including php variables, and deliver that page to the client where the javascript will be executed.

PHP Server-side EX:
<!doctype html>
<html lang="en" class="h-100">
  <body>
		<h1>Hello World</h1>
		<script type="text/javascript">
			var Javascript_variable = <?php echo $php_variable; ?>;
		</script>
	</body>
</html>

JS Client-side EX:
<script type="text/javascript">
	var Javascript_variable = "some string from the above php variabe";
</script>
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