HashTable in PHP

<?php

class SimpleHashing {

   private const BUCKETS_DEFAULT = 16;
   private $buckets;
   private $numBuckets;
   private $size;

   public function __construct($numBuckets = SimpleHashing::BUCKETS_DEFAULT) {
       $this->numBuckets = $numBuckets;
       $this->buckets = array();
       // Initialize an array of size num buckets.
       for($i = 0; $i < $numBuckets; $i++) {
           array_push($this->buckets, null);
       }
   }

   public function isEmpty() {
       return $this->size <= 0;
   }

   // PHP carries no default HashCode() method, so we'll use our own here.
   private function getHash($key) {
       $res = 0;
       $len = strlen($key);

       for($i = 0; $i < $len; $i++) {
           $res = $res * 31 + ord($key[$i]);
       }
       // Mod size, and mask to be positive.
       return ($res & 0x7FFFFFFF) % $this->numbBuckets;
   }

   public function put($key, $value) {
       $bucket = $this->getHash($key);
       $newNode = new HashNode($key, $value);
       if($this->buckets[$bucket]) {
           $this->buckets[$bucket] = $newNode;
           $this->size++;
       }
       else {
           $tmp = $this->buckets[$bucket];
           while($tmp->next) {
               if($tmp->key == $key) {
                   $tmp->value = $value;
                   return;
               }
               $tmp = $tmp->next;
               $this->size++;
           }
       }
   }

   public function get($key) {
       $bucket = $this->getHash($key);
       $tmp = $this->buckets[$bucket];
       while($tmp) {
           if($tmp->key == $key) {
               return $tmp->value;
           }
           $tmp = $tmp->next;
       }
       return null;
   }

   public function contains($key) {
       $bucket = $this->getHash($key);
       $tmp = $this->buckets[$bucket];
       while($tmp) {
           if($tmp->key == $key) {
               return true;
           }
           $tmp = $tmp->next;
       }
       return false;
   }

   public function remove($key) {
       $bucket = $this->getHash($key);
       if(!$this->buckets[$bucket]) {
           return;
       }
       $tmp = $this->buckets[$bucket];
       if($tmp->key == $key) {
           $this->buckets[$bucket] = $this->buckets[$bucket]->next;
           $this->size--;
           return;
       }
       while($tmp->next) {
           if($tmp->next->key == $key) {
               $tmp->next = $tmp->next->next;
               $this->size--;
               return;
           }
           $tmp = $tmp->next;
       }
   }

   public function clear() {
       $this->buckets = array();
       for($i = 0; $i < $this->numBuckets; $i++) {
           array_push($this->buckets, null);
       }
   }

   public function size() {
       return $this->size;
   }

   public function __toString() {
       $res  = "{ ";
       $count = 0;
       for($i = 0; $i < $this->numBuckets; $i++) {
           if($this->buckets[$i]) {
               $tmp = $this->buckets[$i];
               while($tmp) {
                   $res .= $tmp->key . ": " . $tmp->value;
                   $count++;
                   if($count < $this->size) {
                       $res .= ", ";
                   }
                   $tmp = $tmp->next;
               }
           }
       }
       $res .= " }";
       return $res;
   }

}

// Supporting hash node class.
class HashNode {
   public $key;
   public $value;
   public $next;

   public function __construct($key, $value) {
       $this->key = $key;
       $this->value = $value;
       $this->next = null;
   }
}