my custom PHP stdev function VS mysql's STDDEV_POP -

so have code:

$arr = array(   1,2,3,4,5,6 ); $num = count($arr); $sum = array_sum($arr); $average = $sum/$num; foreach($arr $val) {   $sum += pow(($val - $average), 2); } $stdev = sqrt($sum / ($num - 1)); 

versus

select stddev_pop(something) table; 

whereby table

something '1' '2' '3' '4' '5' '6' 

and yet $stdev returns

2.7748873851023 

whereas select returns 1.707825127659933

what's wrong stdev code?

this gave me 1.70

$arr = array(   1,2,3,4,5,6 ); $num = count($arr); $sum2 = 0; $sum = array_sum($arr); $average = $sum/$num; foreach($arr $val) {   $sum2 += pow(($val - $average), 2); } $stdev = sqrt($sum2 / ($num)); echo $stdev;exit;