PHP：为什么类构造函数不起作用？ [关闭]

I created Date() class. But it is not giving the desired outputs.
My code

<?php
class Date{
    public $day = 1;
    public $month = "January";
    public $year = 2013;

    public function __construct($dy, $mon, $yar){
        $this->day = $dy;
        $this->month = $mon;
        $this->year = $yar;
    }
    public function lessthan($dt){
        if($year != $dt->year)
            return $year - $dt->year;
        if($month != $dt->month)
            return $month - $dt->month;
        return $day - $dt->day;
    }
    public function pr(){
        echo $day;
        echo "<br>";
        echo $month;
        return;
    }
}
$a = new Date(1,"January",2002);
$b = new Date(1,"January",2002);
$a->pr();
$b->pr();
echo "Hello";
?>

It only outputs

[newline]
[newline]
Hello

I changed the __construct() to this

public function __construct($dy, $mon, $yar){
        this->$day = $dy;
        this->$month = $mon;
        this->$year = $yar;
    }

But the output is still same. What is the mistake ?

EDIT : sorry for my mistake . i typed this->$day instead of $this->day

You aren't referencing the variables correctly, you need to use

$this->day;
$this->month;
$this->year;

try updating your class to this

class Date{
    public $day = 1;
    public $month = "January";
    public $year = 2013;

    public function __construct($dy, $mon, $yar){
        $this->day = $dy;
        $this->month = $mon;
        $this->year = $yar;
    }
    public function lessthan($dt){
        if($this->year != $dt->year)
            return $this->year - $dt->year;
        if($this->month != $dt->month)
            return $this->month - $dt->month;
        return $this->day - $dt->day;
    }
    public function pr(){
        echo $this->day;
        echo "<br>";
        echo $this->month;
        return;
    }
}

It's your variable references, the $ goes before the first object reference, and not before the member references:

public function __construct($dy, $mon, $yar){
    $this->day = $dy;
    $this->month = $mon;
    $this->year = $yar;
}

You need to reference your class variables correctly. E.g.:

echo $this->day;

This needs to be done with every class variable in your class.

Your OOP is incorrect:

public function __construct($dy, $mon, $yar) {
   $this->day = $dy;
   $this->month = $mon;
   $this->year = $yar;
}

Note the placement of the $ in the assignments.

Your class functions and construct function are missing the $this-> variable to show that they are part of the class and not locally set inside the function.:

public function __construct($dy, $mon, $yar){
    $this->day = $dy;
    $this->month = $mon;
    $this->year = $yar;
}

public function pr(){
    echo $this->day;
    echo "<br>";
    echo $this->month;
    return;
}