staircase.php

<?php

/**
 * Usage : php staircase.php 4
 *
 *    #
 *   ##
 *  ###
 * ####
 */

function drawStaircase(int $n): void
{
    for ($i = 1; $i <= $n; $i++) {
        echo str_pad(str_repeat('#', $i), $n, ' ', STR_PAD_LEFT) . "\n";
    }
}

$n = (int)($argv[1] ?? 0);
if ($n <= 0) {
    throw new \InvalidArgumentException('You must provide a valid "n" number');
}

drawStaircase($n);