Exercise_04.php

<?php

define('EOL', PHP_SAPI == 'cli' ? PHP_EOL : '<br>');

$min = 1;
$max = 10;

$range = [$min, $max];

foreach (range(...$range) as $i) {
    $line = '';
    foreach (range(...$range) as $j) {
        $line .= false              //
        || in_array($i, $range)     // Borders x
        || in_array($j, $range)     // Borders y
        || $i == $j                 // Main diagonal
        || abs($max - $i + 1) == $j // Anti-diagonal
            ? '#' : ' ';
    }
    echo $line.EOL;
}