Sudoku for easy case

Sudoku_easy.php

<?php
/**
 * Created by PhpStorm.
 * User: PC16
 * Date: 30/10/2018
 * Time: 11:59
 */
//var $grid;

Function get_unassigned_location($grid)
{
    for ($row = 0; $row < 9; $row++) {
        for ($col = 0; $col < 9; $col++) {
            //echo "grid [$row] [$col] = " . $grid[$row][$col] . " ";
            if ($grid[$row][$col] == 0) {
                return array($row, $col);
            }

        }
    }
    return array(9, 9);
}

Function used_in_row($grid, $row, $num)
{
    for ($col = 0; $col < 9; $col++){
        //echo $grid[$row][$col] . "($row, $num) <br>";
        if ($grid[$row][$col] == $num) {
            return true;
        }

    }
    return false;
}

Function used_in_col($grid, $col, $num)
{
    for ($row = 0; $row < 9; $row++) {
        //echo "$col, $num <br>";
        if ($grid[$row][$col] == $num) {
            return true;
        }
        }
        return false;

}

Function used_in_box($grid, $box_start_row, $box_start_col,  $num)
{
    for ($row = 0; $row < 3; $row++) {
        for ($col = 0; $col < 3; $col++)
            if ($grid[$row + $box_start_row][$col + $box_start_col] == $num) {
                return true;
            }

    }
    return false;
}

Function is_safe($grid, $row, $col, $num)
{
        // Check if 'num' is not already placed in current row, // current column and current 3x3 box
        $res = ((!used_in_row($grid, $row, $num))
            && (!used_in_col($grid, $col, $num))
            && (!used_in_box($grid, $row - ($row % 3), $col - ($col % 3), $num)));
    return $res;
}

Function solve_soduko($grid)
{
    // If the Soduko grid has been filled, we are done
    //$grid = $soduko;
    if (array(9, 9) == get_unassigned_location($grid)) {echo "<br>";
        displayArray($grid);
        return true; } //
    // Get an unassigned Soduko grid location
    else {
        $row = get_unassigned_location($grid)[0];
        $col = get_unassigned_location($grid)[1];
    }
	// Consider digits 1 to 9
    for ($num = 1; $num <= 9; $num++) {
    // If placing the current number in the current
    // unassigned location is valid, go ahead
        if (is_safe($grid, $row, $col, $num))
    {
        // Make tentative assignment
        //echo "($row, $col, $num)["  . $grid[$row ][$col - 1] . "]";

        $grid[$row][$col] = $num;
        // Do the same thing again recursively. If we go
        // through all of the recursions, and in the end
        // return true, then all of our number placements
        // on the Soduko grid are valid and we have fully
        // solved it

        if (solve_soduko($grid)) {return true; } //
        // As we were not able to validly go through all
        // of the recursions, we must have an invalid number
        // placement somewhere. Lets go back and try a
        // different number for this particular unassigned location
        else $grid[$row][$col] = 0;//echo "[$row][$col] I go back (cause $num)";


    }

}
// If we have gone through all possible numbers for the current unassigned
// location, then we probably assigned a bad number early. Lets backtrack
 // and try a different number for the previous unassigned locations.
return false;
}



function displayArray($arr){
    $ii=3;echo " | ";
    for ($i = 0; $i < 9; $i++){
        for ($j = 0; $j < 9; $j++){

            if($ii == 0) {echo " | "; $ii = 3;}
            $ii--; echo $arr[$i][$j] . " ";
        }
        echo "<br>";
    }

}



$grid = array(
    array(0, 0, 0, 0, 7, 6, 5, 0, 0),
    array(0, 0, 9, 0, 0, 8, 0, 0, 1),
    array(0, 4, 6, 0, 1, 5, 0, 0, 7),

    array(3, 5, 0, 0, 9, 0, 6, 0, 0),
    array(9, 0, 2, 7, 0, 4, 1, 0, 5),
    array(0, 0, 7, 0, 6, 0, 0, 4, 9),

    array(7, 0, 0, 8, 3, 0, 4, 9, 0),
    array(6, 0, 0, 2, 0, 0, 8, 0, 0),
    array(0, 0, 3, 6, 5, 0, 0, 0, 0),
);
/*
 * Complex case
$grid = array(
    array(0, 3, 4, 1, 0, 0, 0, 0, 0),
    array(0, 8, 0, 0, 2, 0, 1, 0, 2),
    array(0, 0, 1, 0, 1, 0, 0, 0, 0),

    array(0, 5, 0, 4, 9, 0, 8, 0, 5),
    array(0, 1, 0, 0, 0, 4, 0, 5, 0),
    array(4, 0, 8, 2, 1, 0, 0, 0, 0),

    array(0, 0, 0, 0, 1, 0, 4, 0, 0),
    array(6, 0, 7, 0, 8, 0, 0, 3, 0),
    array(0, 0, 0, 0, 0, 3, 7, 2, 0),
);
*/

displayArray($grid);
solve_soduko($grid);