custom validation?

<?php
$rand = rand(3,9);
$rand2 = rand(1,7);
$sum = $rand+$rand2;

if (isset($_POST['submit'])) {
$summed = $_POST['sum'];

$compare = substr($_POST['sumval'],4,-2);

$comp = $compare/9;

if ($summed== $comp) {
return true; //somehow to continue with eForm email..etc

}

else { 
return 'some error'; //somehow to stop with eForm and output the error

 }	
 }


?>
<form name="captcha" method="post">
<div><strong>
      <?=$rand?>
      </strong>+<strong>
        <?=$rand2?>
      </strong>=</div><br />

<input type="text" class="textfield" name="sum" id="spam" <?=$borderspam?> /><?=$errspam?>
<br />
<input type="submit" name="submit" id="button" class="submitb" value="Submit" />
<br />
<input type="hidden" name="sumval" id="sumval" value="<?=$sum*9?>09">
      
      
      </form>

function checkField(&$fields,&$vMsg,&$rMsg) {
    if(!empty($fields['Last__Name'])) {
        return false;
    } else {
    return true;
    }
}

//<?php
//create "eformFunctions" snippet with this code

function beforeFormMerge( &$fields ){
	$rand = rand(3,9);
	$rand2 = rand(1,7);
	$sumval = (($rand+$rand2)*9). '09';

	$fields['rand1'] = $rand;
	$fields['rand2'] = $rand2;
	$fields['sumval'] = $sumval;
	
	return true;
}

function formValidate(&$fields, &$vMsg, &$rMsg){
	
	$comp = substr($fields['sumval'],4,-2)/9;
	
	if( $fields['sum'] != $comp ) 
	$vMsg['sum'] = "Anti spam solution is incorrect!";
	
	return true; 

}

return '';
//?>

<div><strong>
      [+rand1+]
      </strong>+<strong>
      [+rand2+]
      </strong>=</div><br />

<input type="text" class="textfield" name="sum" id="spam" />
<br />
<input type="submit" name="submit" id="button" class="submitb" value="Submit" />
<br />
<input type="hidden" name="sumval" id="sumval" value="[+sumval+]"> /

[[eformFunctions]]
[!eForm? &eFormOnBeforeFormMerge=`beforeFormMerge` &eFormOnValidate=`formValidate` ..(more params).. !]