<?php

function a($a, $b, $c) {

    $d = $b * $b - 4 * $a * $c;
    if ($d < 0) return false;

    $x1 = (-$b + sqrt($d)) / (2 * $a);
    $x2 = (-$b - sqrt($d)) / (2 * $a);

    $res = [
        'x1' => $x1,
        'x2' => $x2
    ];

    return $res;
}


echo '<pre>';
var_dump(a(1, 1, -6));
echo '</pre>';