How do I add php code to my forum?

[BBR-APBT]

it would not be echo you would use a variable.

So it would be like this:

<?php
$sql = "select items from table";
$mm = mysql_query($sql);
while ($ms = mysql_fetch_array($mm)) {
$var = "WHAT I WANT ECHOED";
}
?>

Remember to look here to register the variable.
https://vborg.vbsupport.ru/showthread.php?t=228078

Then in the template you would use:

{vb:raw var}