Demonstration of PHP's recursive mkdir() race condition and a PHP-reimplementation that fixes it. Run ./run.sh multiple times to show the issue, then switch out the mkdir implementation and try if you can still reproduce the issue.

mkdir-concurrency-demo.php

<?php
/**
 * Recursively creates the missing parts of a directory path in a manner that is concurrency-safe.
 *
 * @see https://bugs.php.net/bug.php?id=35326
 *
 * @param string $pathname a (nested) directory path to create
 * @param integer $mode the permission to use
 * @return bool true iff the directory path was successfully created
 */
function ensureDirectoryExists($pathname, $mode)
{
    $path_segments = explode(DIRECTORY_SEPARATOR, $pathname);

    $current_pathname = '';
    foreach ($path_segments as $path_segment) {
        $current_pathname = $current_pathname . $path_segment . DIRECTORY_SEPARATOR;
        @mkdir($current_pathname, $mode);
    }

    return is_dir($pathname);
}

$path = 'a/b/c/d/e/f/g/h/i/j/k/l/m/n/o/p/q/r/s/t/u/v/w/x/y/z';

$result = true;
if (!is_dir($path)) {
    // Comment the next line out and the one after that in to test the fix
    $result = mkdir($path, 0777, true);
//     $result = ensureDirectoryExists($path, 0777);
}

if ($result) {
    echo "Success!\n";
} else {
    echo "Failed!\n";
}

run.sh

#!/usr/bin/env bash

NUM_PROCESSES=20

rm -rf ./a

echo "If this doesn't fail on the first try, run it a couple of times. This problem is non-deterministic."

echo "Starting ${NUM_PROCESSES} parallel processes..."

for i in $(seq 1 $NUM_PROCESSES)
do
	php mkdir-concurrency-demo.php  &
done

wait

echo "Finished."