<!-- Collection of code snippets by Arne Vajhøj -->
<!-- posted to eksperten.dk, usenet and other places (2002-now) -->
<?php
function count_prefix($a, $prefix) {
    return array_reduce(array_keys($a), function($carry, $item) use($prefix) { if(strpos($item, $prefix)===0)  return $carry + 1; else return $carry; }, 0);
}

$a = Array( 'navn' => '', 'kommentar' => '', 'email_tjek' => 1, 'valg25' => 1, 'valg225' => 1, 'valg227' => 1, 'mySubmit' => 'Fortsæt');
echo count_prefix($a, 'valg') . "\r\n";
echo count_prefix($a, 'kom') . "\r\n";
echo count_prefix($a, 'foobar') . "\r\n";
?>