To declare a variable in PHP we write the $ sign before the name of the variable :
for example, let's declare a variable named greeting and assign the string Hello to it.
$greeting="Hello"
we can echo out this variable like this: echo "$greeting World".
index.php:
<!doctype html>
<html lang="en">
<head>
<meta charset="UTF-8">
<title>Demo</title>
</head>
<body>
<h1>
<?php
$greeting="Hello";
echo "$greeting World";
?>
</h1>
</body>
</html>