diff --git a/AVL_Tree/avl_tree.php b/AVL_Tree/avl_tree.php
new file mode 100644
index 0000000000..761a4d8284
--- /dev/null
+++ b/AVL_Tree/avl_tree.php
@@ -0,0 +1,192 @@
+<?php
+
+class AVLTree 
+{
+    var $left;
+    var $right;
+    var $depth;
+    var $data;
+   
+    function AVLTree() 
+    {
+        $this->left = NULL;
+        $this->right = NULL;
+        $this->depth = 0;
+        $this->data = NULL;
+    }
+   
+    function balance() 
+    {
+        $ldepth = $this->left !== NULL
+                ? $this->left->depth
+                : 0;
+               
+        $rdepth = $this->right !== NULL
+                ? $this->right->depth
+                : 0;
+               
+               
+        if( $ldepth > $rdepth + 1 ) 
+        { // LR or LL rotation
+            $lldepth = $this->left->left !== NULL
+                     ? $this->left->left->depth
+                     : 0;
+
+            $lrdepth = $this->left->right !== NULL
+                     ? $this->left->right->depth
+                     : 0;
+
+            if( $lldepth < $lrdepth ) 
+            { // LR rotation
+                $this->left->rotateRR(); // consist of a RR rotation of the left child ...
+            } // ... plus a LL rotation of this node, which happens anyway
+
+            $this->rotateLL();
+        } 
+        else if( $ldepth + 1 < $rdepth ) 
+        { // RR or RL rorarion
+            $rrdepth = $this->right->right !== NULL
+                     ? $this->right->right->depth
+                     : 0;
+
+            $rldepth = $this->right->left !== NULL
+                     ? $this->right->left->depth
+                     : 0;
+
+            if( $rldepth > $rrdepth ) 
+            { // RR rotation
+                $this->right->rotateLL(); // consist of a LL rotation of the right child ...
+            } // ... plus a RR rotation of this node, which happens anyway
+
+            $this->rotateRR();
+        }
+    }
+   
+    function rotateLL() 
+    { // the left side is too long => rotate from the left (_not_ leftwards)
+        $data_before =& $this->data;
+        $right_before =& $this->right;
+       
+        $this->data =& $this->left->data;
+        $this->right =& $this->left;
+        $this->left =& $this->left->left;
+        $this->right->left =& $this->right->right;
+        $this->right->right =& $right_before;
+        $this->right->data =& $data_before;
+        $this->right->updateInNewLocation();
+        $this->updateInNewLocation();
+    }
+   
+    function rotateRR() 
+    { // the right side is too long => rotate from the right (_not_ rightwards)
+        $data_before =& $this->data;
+        $left_before =& $this->left;
+       
+        $this->data =& $this->right->data;
+        $this->left =& $this->right;
+        $this->right =& $this->right->right;
+        $this->left->right =& $this->left->left;
+        $this->left->left =& $left_before;
+        $this->left->data =& $data_before;
+        $this->left->updateInNewLocation();
+        $this->updateInNewLocation();
+    }
+   
+    // -- updateInNewLocation
+    // may be overloaded by derived class
+    function updateInNewLocation() 
+    {
+        $this->getDepthFromChildren();
+    }
+   
+   
+    function getDepthFromChildren() 
+    {
+        $this->depth = $this->data !== NULL ? 1 : 0;
+        if( $this->left !== NULL )
+            $this->depth = $this->left->depth+1;
+        if( $this->right !== NULL && $this->depth <= $this->right->depth )
+            $this->depth = $this->right->depth+1;
+    }
+
+    // --- debugging functions
+       
+    function toString() 
+    {
+        $s = "<table border><tr>\n".$this->toTD(0)."</tr>\n";
+        $depth = $this->depth - 1;
+        for( $d = 0; $d < $depth; ++$d ) 
+        {
+            $s .= "<tr>";
+           
+            $s .= $this->left !== NULL
+                ? $this->left->toTD( $d )
+                : "<td></td>";
+               
+            $s .= $this->right !== NULL
+                ? $this->right->toTD( $d )
+                : "<td></td>";
+               
+            $s .= "</tr>\n";
+        }
+       
+        $s .= "</table>\n";
+        return $s;
+    }
+   
+    function toTD( $depth ) 
+    {
+        if( $depth == 0 ) 
+        {
+            $s = "<td align=center colspan=".$this->getNLeafs().">";
+            $s .= $this->data."[".$this->depth."]</td>\n";
+        } 
+        else 
+        {
+            if( $this->left !== NULL ) 
+            {
+                $s = $this->left->toTD( $depth-1);
+            } 
+            else 
+            {
+                $s="<td></td>";
+            }
+
+            if( $this->right !== NULL ) 
+            {
+                $s .= $this->right->toTD( $depth-1);
+            } 
+            else 
+            {
+                if( $this->left !== NULL )
+                    $s.="<td></td>";
+            }
+        }
+       
+        return $s;
+    }
+   
+    function getNLeafs() 
+    {
+        if( $this->left !== NULL ) 
+        {
+            $nleafs = $this->left->getNLeafs();
+           
+            if( $this->right !== NULL )
+                $nleafs += $this->right->getNLeafs();
+            else
+                ++$nleafs; // lus one for the right "stump"
+        } 
+        else 
+        {
+            if( $this->right !== NULL )
+                $nleafs = $this->right->getNLeafs() + 1; // plus one for the left "stump"
+            else
+                $nleafs = 1; // this node is a leaf
+        }
+       
+        return $nleafs;
+    }
+   
+}
+?>