<HTML>
<HEAD>
<TITLE>
</TITLE>
</HEAD>
<BODY>

<?php

// More about variables

$r; echo "r = ", $r, "<BR>"; # No ERROR

$i = 123;
echo $i;
echo "<BR>";
echo "$i";

$a_b__c = "p"; // OK (it contains 3 underscores)

?>

</BODY>
</HTML>
