<?php
    //Var name
    $vars = array("var","echo","test");

    //Var content
    $vars_content = array(5,"lol",'c');

    if(isset($_POST["send"]))
    {
        $message = $_POST['message'];

        for($i=0;$i<count($vars);$i++)
        {
            $message = str_replace("$" . $vars[$i],$vars_content[$i],$message);
        }


        mail($_POST["mail"],$_POST["subject"],$message);
        echo 'mail sent.....';
    }
    else
    {
        echo '<h1>send mail</h1>
        <form action="index.php" method="post">
        to <input type="mail" name="mail"><br />
        subject <input type="text" name="subject"><br />
        message <textarea name="message"></textarea><br />
        <input type="submit" name="send">';
    }
?>