Eight Queens Problem

queens.php

<?php

/**
 * Sample output:
 *
 * $ time php queens.php
 * [ ][ ][ ][ ][ ][X][ ][ ]
 * [ ][ ][X][ ][ ][ ][ ][ ]
 * [ ][ ][ ][ ][ ][ ][X][ ]
 * [ ][X][ ][ ][ ][ ][ ][ ]
 * [ ][ ][ ][ ][ ][ ][ ][X]
 * [ ][ ][ ][ ][X][ ][ ][ ]
 * [X][ ][ ][ ][ ][ ][ ][ ]
 * [ ][ ][ ][X][ ][ ][ ][ ]
 * php queens.php  0.02s user 0.01s system 66% cpu 0.041 total
 */

function random_queen(): array
{
    return ['x' => rand(0, 7), 'y' => rand(0, 7)];
}

function valid($queen1, $queen2): bool
{
    return !(
        $queen1['x'] == $queen2['x']
        || $queen1['y'] == $queen2['y']
        || abs($queen1['x'] - $queen2['x']) == abs($queen1['y'] - $queen2['y'])
    );
}

function coords($queen): string
{
    return $queen['x'] . ',' . $queen['y'];
}

$queens = $exclude = [];

while (count($queens) < 8) {
    do {
        $new_queen = random_queen();
    } while (isset($exclude[coords($new_queen)]));

    $valid = true;
    foreach ($queens as $current_queen) {
        if (!valid($new_queen, $current_queen)) {
            $valid = false;
            break;
        }
    }

    if ($valid) {
        $queens[coords($new_queen)] = $new_queen;
    }
    $exclude[coords($new_queen)] = true;

    // All possible positions have been tried, there is no valid solution that
    // includes the placed queens, start over
    if (count($exclude) == 64) {
        $queens = $exclude = [];
    }
}

$output = array_fill(0, 8, array_fill(0, 8, ' '));
foreach ($queens as $queen) {
    $output[$queen['x']][$queen['y']] = 'X';
}

foreach (range(0, 7) as $x) {
  foreach (range(0, 7) as $y) {
    echo '[' . $output[$x][$y] . ']';
  }
  echo PHP_EOL;
}