Find intersection of lines

<?php

// Check the direction these three points rotate
// Therefore I use this awesome way @Peter Wone[1] wrote. It is really well explained by @Tom[2].
// [1] http://stackoverflow.com/a/16725715/3001970
// [2] http://stackoverflow.com/a/30160064/3001970
function RotationDirection($p1x, $p1y, $p2x, $p2y, $p3x, $p3y) {
    if ((($p3y - $p1y) * ($p2x - $p1x)) > (($p2y - $p1y) * ($p3x - $p1x))) return 1;
    elseif ((($p3y - $p1y) * ($p2x - $p1x)) == (($p2y - $p1y) * ($p3x - $p1x))) return 0;
    else return -1;
}

// Checks if two line have a intersection
function checkIntersection($p1x, $p1y, $p2x, $p2y, $p3x, $p3y, $p4x, $p4y) {

    // Determine the direction they rotate. (You can combine this all into one step.)
    $face1CounterClockwise = RotationDirection($p1x, $p1y, $p2x, $p2y, $p4x, $p4y);
    $face2CounterClockwise = RotationDirection($p1x, $p1y, $p2x, $p2y, $p3x, $p3y);
    $face3CounterClockwise = RotationDirection($p1x, $p1y, $p3x, $p3y, $p4x, $p4y);
    $face4CounterClockwise = RotationDirection($p2x, $p2y, $p3x, $p3y, $p4x, $p4y);

    // If face 1 and face 2 rotate different directions and face 3 and face 4 rotate different directions,
    // then the lines intersect.
    $intersect = ($face1CounterClockwise !== $face2CounterClockwise &&
                  $face3CounterClockwise !== $face4CounterClockwise);

    // If lines are on top of each other.
    if ($face1CounterClockwise == false && $face2CounterClockwise == false &&
        $face3CounterClockwise == false && $face4CounterClockwise == false)
        $intersect = true;

    return $intersect;

}

// if the lines intersect, the result contains the x and y of the intersection
// modifed from soure: jsfiddle.net/justin_c_rounds/Gd2S2/
function checkLineIntersection($line1StartX, $line1StartY, $line1EndX, $line1EndY,
                               $line2StartX, $line2StartY, $line2EndX, $line2EndY) {


    $result = array ( 'x' => 0,
                     'y' => 0);

    $denominator = (($line2EndY - $line2StartY) * ($line1EndX - $line1StartX)) -
        (($line2EndX - $line2StartX) * ($line1EndY - $line1StartY));
    if ($denominator == 0) {
        return $result;
    }

    $a = $line1StartY - $line2StartY;
    $b = $line1StartX - $line2StartX;
    $numerator1 = (($line2EndX - $line2StartX) * $a) - (($line2EndY - $line2StartY) * $b);
    $numerator2 = (($line1EndX - $line1StartX) * $a) - (($line1EndY - $line1StartY) * $b);
    $a = $numerator1 / $denominator;
    $b = $numerator2 / $denominator;

    // if we cast these lines infinitely in both directions, they intersect here:
    $result['x'] = $line1StartX + ($a * ($line1EndX - $line1StartX));
    $result['y'] = $line1StartY + ($a * ($line1EndY - $line1StartY));

    $result['x'] = round($result['x'], 3);
    $result['y'] = round($result['y'], 3);
    return $result;

};


// check if point x y exist in array
function point_exists ($point, $points){
    $return = false;
    foreach($points as $p){
        if ($p['x'] == $point['x'] && $p['y'] == $point['y'])
            $return = true;
    }
    return $return;
}



$list_lines = array (
    array (715.341, 0, 757.297, 600),
    array (0, 249.169, 800, 179.178),
    array (0, 256.196, 800, 186.205),
    array (0, 284.225, 800, 200.142),
    array (396.716, 0, 481.041, 600),
    array (0, 311.374, 800, 227.29),
    array (0, 355.76, 800, 229.053),
    array (0, 521.525, 800, 437.442),
    array (696.134, 0, 611.809, 600)
);

$line1 = 0;
$line2 = 0;
$points = array();

foreach ($list_lines as $f){
    $line2 = 0; // reset counter of secound loop
    foreach ($list_lines as $s){
        // only check intersection if line1 and line2 are not the same
        if (implode($f) !== implode($s)) {
            // check if intersection exists
            $intesect = checkIntersection($f[0], $f[1], $f[2], $f[3], $s[0], $s[1], $s[2], $s[3]);
            if ($intesect) {
                // get point x,y
                $point = checkLineIntersection ($f[0], $f[1], $f[2], $f[3], $s[0], $s[1], $s[2], $s[3]);
                // check if point exists
                if (!point_exists($point, $points)) {
                    // insert point into array and write line IDs
                    array_push($point, $line1, $line2);
                    array_push($points, $point);
                }
            }
        }
        $line2++;
    }
    $line1++;
}

// output
echo "<pre>";
foreach ($points as $x => $point ){
    echo "Point $x. ". implode($point, ", ") ."\n";
}
echo "</pre>";
?>