PHP function to calculate standard deviation

// Function to calculate square of value - mean
function sd_square($x, $mean) { return pow($x - $mean,2); }

// Function to calculate standard deviation (uses sd_square)    
function sd($array) 
{
    if(count($array)> 0):
    return sqrt(
            array_sum(array_map("sd_square", 
                            $array, 
                            array_fill
                                    (
                                        0,
                                        count($array), 
                                        (array_sum($array) / count($array)) 
                                    ) 
                                ) 
                    ) / (count($array)-1) );
    endif;
}