Solution to Codility problem https://codility.com/programmers/task/falling_disks

solution.php

<?php
function solution($A, $B) {
    // Build an array relating maximum circumferences to depths
    $smallest_at = [-1 => PHP_INT_MAX];
    
    for ($i = 0; $i < count($A); $i++) {
        $smallest_at[$i] = min($smallest_at[$i - 1], $A[$i]);
    }
    
    // Start dropping
    $dropped = 0;
    
    // Reverse our way through the $smallest_at array and attempt to drop
    // where possible
    for ($i = count($A) - 1; $i >= 0 && $dropped < count($B); $i--) {
        if ($smallest_at[$i] >= $B[$dropped]) {
            $dropped++;
        }
    }
    
    return $dropped;
}