A good function to help to find if a number is odd or pair.
function is_odd($tmpnumber){
if ($tmpnumber < '0') $tmpnumber = gmp_mod($tmpnumber);
if ($tmpnumber == '0') return(false);
elseif ($tmpnumber == '1') return(true);
elseif ($tmpnumber > '1'){
if (gmp_div_q($tmpnumber,'2') != '0') return(true);
else return(false);
}
}