Morse code algorithm in PHP

morse.php

<? php
  class MorseNode {
  private $value;
  private $leftNode;
  private $rightNode;
  
  function __construct($value, $leftNode, $rightNode){
    $this->value = $value;
    $this->leftNode = $leftNode;
    $this->rightNode = $rightNode;
  }
  
  function getLeftNode(){
    return $this->leftNode;
  }
  
  function getRightNode(){
    return $this->rightNode;
  }
  
  function getValue(){
    return $this->value;
  }
}

function traverse($node, $signalsList, &$result){
   //exit conditions
   if(count($signalsList) == 0 ){
     array_push($result,$node[0]);
     return;
   }
  
   if($node[1] == null and $node[2] == null){
     array_push($result,$node[0]);
     return;
   }
  
  //normal flow
   $signalBit = array_shift($signalsList);
   switch($signalBit){
     case ".": //left, 1 index
       if($node[1] != null){ //leaf check
         traverse($node[1],$signalsList, $result);
       }
       break;
     case "-": //right, 2 index
       if($node[2] != null){ //leaf check
         traverse($node[2],$signalsList, $result);
       }
       break;
     case "?":
       if($node[1] != null){ //leaf check
         traverse($node[1],$signalsList, $result);
       }
       if($node[2] != null){ //leaf check
         traverse($node[2],$signalsList, $result);
       }
       break;
   }
   

}
function possibilities ($signals) {
  
  // the class declared above MorseNode can be used to make a more verbose but a bit slow version of the tree
  // this Array form is a compact version, I will use this for efficiency reason
  $morseTree = ["start",["E",["I",["S",null,null],["U",null,null]],["A",["R",null,null],["W",null,null]]],["T",["N",["D",null,null],["K",null,null]],["M",["G",null,null],["O",null,null]]]];
  $signalList = str_split($signals);
  $result = [];
  traverse($morseTree, $signalList, $result);
  return $result;
   
}