undefined for me, thus I've implemented my own correlation which is much faster and simpler than the one provided above.
function Corr($x, $y){
$length= count($x);
$mean1=array_sum($x) / $length;
$mean2=array_sum($y) / $length;
$a=0;
$b=0;
$axb=0;
$a2=0;
$b2=0;
for($i=0;$i<$length;$i++)
{
$a=$x[$i]-$mean1;
$b=$y[$i]-$mean2;
$axb=$axb+($a*$b);
$a2=$a2+ pow($a,2);
$b2=$b2+ pow($b,2);
}
$corr= $axb / sqrt($a2*$b2);
return $corr;
}