Consider the following <input> html element.

<input type='file' id='file_input'></input>

<?php
  // getting the input element
  $file_input = $driver->findElement(
    WebDriverBy::id('file_input')
  );

  // set the file detector
  $file_input->setFileDetector(new LocalFileDetector());

  // upload the file and submit the form
  $file_input->sendKeys($file_path)->submit();

If you get an error PHP Fatal error: Uncaught exception 'UnknownServerException' with message 'Expected there to be only 1 file. There were: 0, try to send the file name as an array. There is a bug in the PHP library. It will issue a warning, but it will work in Chrome.

$input->sendKeys(array($file));

PHP Warning: is_file() expects parameter 1 to be a valid path, array given in php-webdriver/lib/remote/LocalFileDetector.php on line 23