validating textarea using php

hi

i need to validate textarea of a html form using php

<textarea name="comments" cols="26" rows="3" id="comments"><?php
echo($comments);?></textarea>

presently my php code to validate the text area is

if($comments == "" )
{
$error.="<br>Please enter your comments";
}

with this code if a user hits the space bar once or couple of times as a
matter of fact there are no characters entered by the user i do not want
this to happen, if a user simply hits the spacebar and does not type
anything i should be able to display an alert message.

please advice how i can change the above php code.

thanks.