A small implementation of Tic-Tac-Toe in PHP

tic-tac-toe.php

<?php

const PLAYER_1 = 'X';
const PLAYER_2 = 'O';

function format_grid(array $grid): string {
    return join(
        array_map(
            fn($grid_str_line) => "\t{$grid_str_line}\n",
            array_map(
                fn (array $grid_list_line): string => join('|', $grid_list_line),
                array_chunk($grid, 3)
            )
        )
    );
}

function is_index_played(string $value): bool {
    return $value === PLAYER_1 || $value === PLAYER_2;
}

function is_grid_full(array $grid): bool {
    return array_sum(
        array_map(
            is_index_played(...),
            $grid
        )
    ) === 9;
}

function is_grid_won(array $grid): bool {
    for ($i = 0; $i < 9; $i += 3) {
        /* Check rows */
        if ($grid[$i] === $grid[$i+1] && $grid[$i] === $grid[$i+2]) {
            return true;
        }
        /* Check columns */
        $j = $i % 3;
        if ($grid[$j] === $grid[$j+3] && $grid[$j] === $grid[$j+6]) {
            return true;
        }
    }
    /* Check diagonals, which are either 1 or 4 away from the centre */
    $center = 4;
    return ($grid[$center] === $grid[$center-1] && $grid[$center] === $grid[$center+1])
        || ($grid[$center] === $grid[$center-4] && $grid[$center] === $grid[$center+4]);
}

/* Use numeric string semantics to parse the input */
function parse_input(int $string) {
    return $string-1;
}

function toggle_player(string $player): string {
    return $player === PLAYER_1 ? PLAYER_2 : PLAYER_1;
}

$player = PLAYER_1;
$grid = range('1', '9');

echo "Welcome to Tic-Tac-Toe!
The rules are simple, first person to align 3 symbols of their choice wins.
Select a cell by typing in the number of it.
Good luck!\n";

do {
    echo "Grid status:\n", format_grid($grid);

    retry:
    $raw_input = readline("Player {$player} which cell do you want to play?\t");
    try {
        $input = parse_input($raw_input);
    } catch (TypeError) {
        echo "Please enter a number\n";
        goto retry;
    }

    if (is_index_played($grid[$input])) {
        echo "The cell you have chosen is already taken\n";
        goto retry;
    }

    $grid[$input] = $player;
    if (is_grid_won($grid)) {
        echo "Congratulations player $player has won!\n";
        exit(0);
    }
    $player = toggle_player($player);
} while (!is_grid_full($grid));

echo "Sadly no one has won this round\n";
echo "Grid status:\n", format_grid($grid);

exit(0);