PHP-Code

<?php

$checker = function(string $word, string $trunk): bool {
  // validate input
  if (!preg_match('/[a-z]+/', $word)) return false;
  if (!preg_match('/[a-z]+/', $trunk)) return false;
  
  // for later usage convert string to array
  $word = str_split($word);
  $trunk = str_split($trunk);

  foreach ($word as $character) {
  	// search letter in trunk
    if (($key = array_search($character, $trunk)) !== false) {
      unset($trunk[$key]);
      $trunk = array_values($trunk);
    }
    // no letter found in trunk
    else {
      return false;
    }
  }
  
  return true;
};

var_dump($checker('', '')); // false, input not valid
var_dump($checker('ABC', 'ABC')); // false, input not valid
var_dump($checker('abc', 'cba')); // true
var_dump($checker('abcd', 'cba')); // false, 'd' missing in trunk
var_dump($checker('abc', 'dcba')); // true
var_dump($checker('aabbcc', 'abcabc')); // true
var_dump($checker('mama', 'mneamna')); // true
var_dump($checker('papa', 'aapq')); // false
var_dump($checker('aabaacbaacbb', 'bbaabaaccaaa')); // false