linear_regression.php

<?php

ini_set('display_errors',1);
ini_set('display_startup_errors',1);
error_reporting(E_ALL);


$x = array(1,2,3,4,5,6,7,8,9);
$y = array(3,4,3,5,6,8,7,9,9);

print_r(linear_regression($x,$y));
exit;

/**
 * linear regression function
 * @param $x array x-coords
 * @param $y array y-coords
 * @returns array() m=>slope, b=>intercept
 */
function linear_regression($x, $y) {

		// calculate number points
		$n = count($x);

		// ensure both arrays of points are the same size
		if ($n != count($y)) {

				trigger_error("linear_regression(): Number of elements in coordinate arrays do not match.", E_USER_ERROR);

		}

		// calculate sums
		$x_sum = array_sum($x);
		$y_sum = array_sum($y);

		$xx_sum = 0;
		$xy_sum = 0;

		for($i = 0; $i < $n; $i++) {

				$xy_sum+=($x[$i]*$y[$i]);
				$xx_sum+=($x[$i]*$x[$i]);

		}

		// calculate slope
		$m = (($n * $xy_sum) - ($x_sum * $y_sum)) / (($n * $xx_sum) - ($x_sum * $x_sum));

		// calculate intercept
		$b = ($y_sum - ($m * $x_sum)) / $n;

		// return result
		return array("m"=>$m, "b"=>$b);

}