Here is my solution:

<?php
 
$train = explode(" ", readline());
$capacity = readline();
 
$input = readline();
 
while ($input != "end") {
    $args = explode(" ", $input);
    $command = $args[0];
 
    if ($command == "Add") {
        $wagon = $args[1];
        array_push($train, $wagon);
    } else {
        for ($i = 0; $i < count($train); $i++) {
            if ($command + $train[$i] <= $capacity) {
                $train[$i] += $command;
                break;//the break; is needed: otherwise it will continue adding to the other wagons of the train as well
            }
        }
    }
 
    $input = readline();
}
 
echo implode(" ", $train);