Simple(?) Validation

I was wondering if someone could assist me in some coding.

I am attempting to take a textbox input and check to see if it is valid.

I am meaning:

input textbox must have either: abc, def, ghi or jkl.

I want to be able to make sure that someone has entered one of them, all is good BUT if not, then kick up a fuss.

I use the following to check for another textbox:

if (empty($username)) $error1= $error1. "<li><font color=red>You need to inform us what you would like your '<B>username</B>' to be!</font>";

I would like the check for the other variable to be able to kick back the $error1 also if it does not match one of the variables listed above.

Thanks all for your help in the matter.

if(!in_array($_GET['variablename'],array('abc','def','ghi','jkl')) {
  $error1.=$error1.'New error message';
}

HTH
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