I cannot add custom php code to registration template ?

[Mean]

I want to add some custom php codes to registration template (or page).
But when I attempt to add this

<?
My php codes
?>

in admincp > in register template, at the end of the codes, it says:

Parse error: syntax error, unexpected T_ENCAPSED_AND_WHITESPACE, expecting T_STRING or T_VARIABLE or T_NUM_STRING in /usr/hosts/domains/propaket/xxx_pro/www.xxx.net/httpdocs/includes/adminfunctions_template.php(3596) : eval()'d code on line 310

So where should I add my php codes ?