<?
$text="aaaaaaaaaa";
$len=strlen($text);
if ($len < "10")
{
?>
<form method="POST" action="">
<input type="submit" name="submit" value="Давай" disabled>
</form>
<? }
else
{
?>
<form method="POST" action="">
<input type="submit" name="submit" value="Давай" >
</form>
<? } ?>