Using breadth-first search to solve 8-puzzle

Question

I am trying to solve 8 puzzle problem using breadth first search in PHP, but I keep on getting an error. I tried to find what's the syntax error for the error showing T_Variable in my code, I don't see one.

Parse error: syntax error, unexpected '$this' (T_VARIABLE) in C:\xampp\htdocs\8BFS.php on line 9

My code is

<?php

class Puzzle {
var $pos;
var $sequence;
var $depth;

function  __construct($current_pos) { 
    $this->pos = $current_pos;
    $this->sequence = array();
    $this->depth=1; 
}
function goalTest($goal) {     
    if ($this->pos === $goal) {       
        return True;        
    } else {
        return False;        
    } 
}
function possibleMoves() {     
    $Moves = array();     
    for ($i = 0; $i < 3; $i++) {   
        for ($j = 0; $j < 3; $j++) {   
            if ($this->pos[$i][$j] == 0) {  
                break 2;                
            }  
        }     
    }
    $this->checkMove($i, $j, $i - 1, $j, $Moves);   
    $this->checkMove($i, $j, $i + 1, $j, $Moves);  
    $this->checkMove($i, $j, $i, $j - 1, $Moves);
    $this->checkMove($i, $j, $i, $j + 1, $Moves);
    return $Moves;
}
function moveBlank($srcRow, $srcCol, $destRow, $destCol) {   
    $newpos = $this->pos;    
    $tmp = $newpos[$destRow][$destCol];
    $newpos[$destRow][$destCol] = $newpos[$srcRow][$srcCol]; 
    $newpos[$srcRow][$srcCol] = $tmp;
    return $newpos;  
}
function InSequence($pos) { 
    for ($i = 0; $i < count($this->sequence); $i++) { 
        if ($this->sequence === $pos) {        
            return TRUE;           
        }        
    }     
    return FALSE;
}
function canMove($srcRow, $srcCol, $destRow, $destCol) { 
    if ($srcRow < 0 or $srcCol < 0 or $destRow < 0 or $destCol < 0) {  
        return FALSE;        
    } 
    if ($srcRow > 2 or $srcCol > 2 or $destRow > 2 or $destCol > 2) { 
        return FALSE;        
    } 
    return TRUE;    
}
function checkMove($srcRow, $srcCol, $destRow, $destCol, & $Moves) {      
    if ($this->canMove($srcRow, $srcCol, $destRow, $destCol)) { 
        $newpos = $this->moveBlank($srcRow, $srcCol, $destRow, $destCol);      
        if (!$this->InSequence($newpos)) {       
            $newMove = new Puzzle($newpos);      
            $newMove->sequence = array_merge($this->sequence);
            $newMove->sequence[] = $newpos;           
            $Moves[] = $newMove;           
        }
    }   
}
function printSequence() {
    for ($i = 0; $i < count($this−>sequence); $i++) {
        print ("Step $i −−−−−−−<br>   ");
        $this−>printPos($this−>sequence[$i]);
        print("<br>");
    }
}
function printPos($pos) {
    for ($i = 0; $i < 3; $i++) {
        for ($j = 0; $j < 3; $j++) {
            print(" " . $pos[$i][$j] . " ");
        }
        print("<br>");
    }
}
}

$start_time = microtime();

$initial_pos = array(
    array(2, 8, 3),
    array(1, 6, 4),
    array(7, 0, 5)
);

$goal_pos = array(
    array(1, 2, 3),
    array(8, 0, 4),
    array(7, 6, 5)
);

$initial_state = new Puzzle($initial_pos);
$initial_state−>sequence[] = $initial_pos;

$nodeQue = new SplQueue();
$nodeQue−>enqueue($initial_state);
$nodeQue−>rewind();
$i = 1;

while ($nodeQue−>valid()) {
    $i++;
    if ($i % 10000 == 0) {
        print("$i steps have been finished<br>");
        print("Unopened Nodes left in the Que = " . $nodeQue−>count() . "<br>");
        $end_time = microtime();
        $exec_time = $end_time − $start_time;
        print ("Time executed to $i steps is $end_time <br>");
    }
    $current_state = new Puzzle();
    $current_state = $nodeQue−>dequeue();
    if ($current_state−>goalTest($goal_pos) == TRUE) {
        print("Solution found in $i steps<br>");
        print("Unopened Nodes left in the Que=" . $nodeQue−>count() . "<br>");
        $current_state−>printSequence();
        break;
    }

    $moves = $current_state−>possibleMoves();
    foreach ($moves as $move) {
        $nodeQue−>enqueue($move);
    }
    $nodeQue−>rewind();
}


print ("<br>Maximum Memory used " . memory_get_peak_usage());
print ("<br>Current Memory used" . memory_get_usage());
$end_time = microtime();
$time_exec = $end_time − $start_time;
print("<br>Execution time used =" . $time_exec);

?>

I'm stuck at the beginning only, I couldn't find y that error keeps on coming. It used to be unexpected ' ' error , after I removed all the extra spaces, now it shows unexpected '$this'.

What might be the syntax error here, I asked my friend to run the same program and she says it runs perfectly fine and she says she has the same php version installed, I have this installed:

wampserver2.5-Apache-2.4.9-Mysql-5.6.17-php5.5.12-64b 

Answer 1

This code worked for me. However, I spot a potential issue on line 120:

$current_state = new Puzzle();

You are calling a constructor which isn't implemented. So some of your variables are not getting set.