Function that tests if the suduko is solvedor no

Soduko_tester.php

<?php
/**
 * Created by PhpStorm.
 * User: Tounsi
 * Date: 30/10/2018
 * Time: 11:59
 */

Function used_in_row($grid, $row, $num)
{
    $used = 0;
    for ($col = 0; $col < 9; $col++){
        if ($grid[$row][$col] == $num)  { $used++;}
        if ($used > 1) {
            return true;
        }
    }
    return false;
}

Function used_in_col($grid, $col, $num)
{
    $used = 0;
    for ($row = 0; $row < 9; $row++){
        if ($grid[$row][$col] == $num) { $used++;}
    if ($used > 1) {
        return true;
    }
   }
   return false;
}

Function used_in_box($grid, $box_start_row, $box_start_col,  $num)
{
    $used = 0;
    for ($row = 0; $row < 3; $row++) {
        for ($col = 0; $col < 3; $col++)
        {
            $used = 0;
            if ($grid[$row + $box_start_row][$col + $box_start_col] == $num) {
                $used++;
            }
        if ($used > 1) {
            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 displayArray($arr){
    for ($i = 0; $i < 9; $i++){
        for ($j = 0; $j < 9; $j++){
            if (($j)%3 == 0) {echo "|";}
            echo $arr[$i][$j] . " ";
        }
        if (($i+1)%3 == 0) {echo "<br>----------------------";}
        echo "<br>";
    }
}

function testSudoku($arr){
    for ($row = 0; $row < 9; $row++) {
        for ($col = 0; $col < 9; $col++) {
            if (($arr[$row][$col] == 0) )  {
                echo "[$row][$col] = zero";
                return false;
                }
            if (!(is_safe($arr, $row, $col, $arr[$row][$col])))  {
                echo "[$row][$col] = " . $arr[$row][$col] . " (Value used more than once!)";
                return false;
            }
        }
    }
    return true;
}

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

displayArray($grid);

$res = testSudoku($grid);

echo "<br>";

if ($res){
    echo "Complete";
    }
    else{
    echo  "No ";
}