Alphabet and numbers key generator

alphaKeyGen.php

<?php

class AlphaKeyGen {

  private $length;
  private $alphaData, $key = [];

  public function __construct($length = 5) {
    $this->length = $length;
    $this->alphaData = array_merge(
      range('a', 'z'), 
      range(0, 9)
    );
  }

  public function next() {
    if (!count($this->key)) $this->initKey();
    else $this->initNew();
    return $this;
  }

  public function set($key = null) {
    if (!is_null($key)) {
      $this->key = str_split($key);
      $this->length = count($this->key);
    }
    return $this;
  }

  public function get() {
    return implode('', $this->key); 
  }

  private function initKey() {
    for ($i = 0; $i < $this->length; ++$i) {
      $this->key[] = $this->alphaData[0];
    }
  }

  private function initNew() {

    for ($i = $this->length - 1; $i >= 0; --$i) {

      $currentKey = array_search(
        $this->key[$i],
        $this->alphaData
      );

      if ($currentKey === (count($this->alphaData) - 1)) {
        $this->key[$i] = $this->alphaData[0];
      }

      if ($currentKey < (count($this->alphaData) - 1)) {
        $this->key[$i] = $this->alphaData[$currentKey + 1];
        break;
      }
    }
  }
}


// example
echo (new AlphaKeyGen)->set()->next()->get()."\n"; // result: aaaaa
echo (new AlphaKeyGen)->set('aaaaa')->next()->get()."\n"; // result: aaaab
echo (new AlphaKeyGen)->set('aaaa9')->next()->get()."\n"; // result: aaaba
echo (new AlphaKeyGen)->set('8999999')->next()->get()."\n"; // result: 9aaaaaa