php类到Json期望只返回设定值

Question

I have a class with multiple properties, all of which are optional, but can accept empty or null as a value. When converting the class to a JSON object, I require it to return only those properties of the class which are being set.

I can't filter then the final result since the value can be null or empty which is a valid entry. So in short, I want only those properties for whose setter functions had been called.

<?php    
class MyClass{

public $property1;
public $property2;
public $property3;
public $property4;
public $property5;

public function setProperty1($property1){
    $this->property1 = $property1;
    return $this;
}

public function setProperty2($property2){
    $this->property2 = $property2;
    return $this;
}

public function setProperty3($property3){
    $this->property3 = $property3;
    return $this;
}

public function setProperty4($property4){
    $this->property4 = $property4;
    return $this;
}

public function setProperty5($property5){
    $this->property5 = $property5;
    return $this;
}
}

$obj = new MyClass();
$obj->setProperty1("p1");
$obj->setProperty2("");
$obj->setProperty3(null);
echo json_encode($obj);

Output:{"property1":"p1","property2":"","property3":null,"property4":null,"property5":null}

Expected: {"property1":"p1","property2":"","property3":null}