All PHP code goes between the php tag. It starts with <?php and ends with ?>. 

The default file extension for PHP files is “.php”.

Every PHP statement end with a semicolon (;) 

The syntax of PHP tag is given below:

<?php   
echo "Hello, world!";
?>

Output:

Hello, world!

Embedding PHP within HTML-

PHP code is embedded within the HTML document using <?php to start the PHP code block and ?> to end it. 

<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <title>A Simple PHP File</title>
</head>
<body>
    <h1><?php echo "Hello, world!"; ?></h1>
</body>
</html>

Output:

Hello, world!