//get pearsons correlation coeficient

function pearsons_correlation($x,$y){

    if(count($x)!==count($y)){return -1;}   

    $x=array_values($x);

    $y=array_values($y);    

    $xs=array_sum($x)/count($x);

    $ys=array_sum($y)/count($y);    

    $a=0;$bx=0;$by=0;

    for($i=0;$i<count($x);$i++){     

        $xr=$x[$i]-$xs;

        $yr=$y[$i]-$ys;     

        $a+=$xr*$yr;        

        $bx+=pow($xr,2);

        $by+=pow($yr,2);

    }   

    $b = sqrt($bx*$by);

    if($b==0) return 0;

    return $a/$b;

}

​

echo pearsons_correlation(array(1, 3, 7), array(1, 4, 9));//0.99890610723867