php 实现开根号，不能用sqrt 函数

/**

 *  $t 是误差范围值

 */
function squre($number ,$t){
$x1 = $number;
$x2 = $number/2;
while( abs($x1 - $x2) > $t){
$x1 = $x2;
$x2 = ($x1 + $number/$x1)/2;
}
return $x2;
}

echo squre(5,0.1);