class A
{
	public function __construct()
	{
		echo 'building an object';
	}
	
	public function __destruct()
	{
		echo 'destroying the object';
	}
}

$obj = new A();

Well, as you can not call the constructor explicitly:

$obj->__construct();

So we should not call the destructor explicitly:

$obj->__destruct();

The problem is that I’ve seen this many times, but it’s a pity that this won’t destroy the object and it is still a valid PHP code.

Constructors and destructors in PHP are part of the so called magic methods. Here’s what the doc page says about them.

The function names __construct(), __destruct(), __call(), __callStatic(), __get(), __set(), __isset(), __unset(), __sleep(), __wakeup(), __toString(), __invoke(), __set_state() and __clone() are magical in PHP classes. You cannot have functions with these names in any of your classes unless you want the magic functionality associated with them.

To be more precise let’s take a look of the definition of destructors.

PHP 5 introduces a destructor concept similar to that of other object-oriented languages, such as C++. The destructor method will be called as soon as there are no other references to a particular object, or in any order during the shutdown sequence.

What if I Call the Destructor Explicitly?

Let’s see some examples!

class A
{
	public function __destruct()
	{
		echo 'destroying the object';
	}
}

$obj = new A();

// prints hello world
echo 'hello world';

// PHP interpreter stops the script execution and prints
// 'destroying the object'

This is actually a normal behavior. At the end of the script the interpreter frees the memory. Actually every object has a built-in destructor, just like it has built-in constructor. So even we don’t define it explicitly, the object has its destructor. Usually this destructor is executed at the end of the script, or whenever the object isn’t needed anymore. This can happen, for instance, at the end of a function body.

Now if we call the destructor explicitly, which as I said I’ve seen many times, here’s what happen.

class A
{
	public function __destruct()
	{
		echo 'destroying the object';
	}
}

$obj = new A();

// this is valid and it prints 'destroying the object'
// BUT IT DOES NOT DESTROY THE OBJECT
$obj->__destruct();

// prints hello world
echo 'hello world';

// HERE PHP ACTUALLY DESTROYS THE $obj OBJECT
// ... and again prints 'destroying the object'

As you can see calling the destructor explicitly doesn’t destroy the object. So the question is …

How to Destroy an Object Before the Script Stops?

Well, to destroy an object you can assign a NULL value to it.

class A
{
	public function __destruct()
	{
		echo 'destroying the object';
	}
}

$obj = new A();

// prints 'destroying the object'
$obj = null;

// prints 'hello world'
echo 'hello world';

// the script stop its execution

Caution

Be aware of that if you don’t clone the object $obj, and simply assign it to another variable, then $obj = null will be pointless. Let’s see the following example.

class A 
{
	public function printMsg()
	{
		echo 'I still exist';
	}
	
	public function __destruct()
	{
		echo 'destroying the object';
	}

}

$obj = new A();

// $newObj is pointing to $obj
$newObj = $obj;

// this doesn't destroy $newObj
// as it appears both $obj and $newObj point to the same memory
// so PHP doesn't free this memory
$obj = null;

// prints 'i still exist'
$newObj->printMsg();

// prints 'hello world'
echo 'hello world';

// now the scripts destroys the "object", which in this
// case is $newObj and prints 'destroying the object'

This example shows us that actually by assigning NULL to an object doesn’t quite destroy it if there are another objects pointing to the same memory.

class A 
{
	public function printMsg()
	{
		echo 'I still exist';
	}
	
	public function __destruct()
	{
		echo 'destroying the object';
	}

}

$b = new A();

$d = $c = $b;

$b = null;

$c->printMsg();
$d->printMsg();

// prints 'destroying the object' ONLY ONCE

In this last example there are two interesting things to note. First $b = null doesn’t call the destructor, and at the end of the script there’s only one implicit call of the destructor, although there are two objects.

Conclusion

The important thing to note is that you shouldn’t call the destructor of an object explicitly! Not because it will throw an fatal error, but simply because it won’t destroy the object.

[1] PHP: Constructors and Destructors

The question is whether PHP cannot help us with built-in functions to perform these calculations for us. It is obvious, that from version 5.3.0 and later, there is an OOP section, which is great, but unfortunately this version is still not updated everywhere. So, how to accomplish the task?

The Wrong Approach

As I said, there are many ways to go in the wrong direction. One of them is to subtract 30 days from current date. This is completely wrong, because not every month has 30 days. Here, some developers will begin to predefine arrays to indicate the number of days in each month, which then will be used in their complicated calculations. Here is an example of this wrong approach.

echo date('Y-m-d', strtotime(date('Y-m-d')) - 60*60*24*30);

This line is full of mistakes. First of all strtotime(date(‘Y-m-d’)) can be replaced by the more elegant strtotime(‘now’), but for this later. Another big mistake is that 60*60*24*30, which is number of seconds in 30 days can be predefined as a constant. Eventually the result is wrong, because not every month has 30 days.

The Correct Approach

A small research of the problem and the functions in versions prior of 5.3.0 of PHP is needed. Typical case study of date formatting happen when working with dates from a database. The following code is a classical example.

// 2008 05 23, 2008-05-23 is stored into the DB
echo date('Y m d', strtotime('2008-05-23'));

// 2008 May 23
echo date('Y F d', strtotime('2008-05-23'));

The problem, perhaps, is that too often strtotime() is used like this, with exactly this type of strings. However much more interesting is that strtotime() can do much more.

strtotime() Can Do Much More

Let us first look at the documentation of this function. What parameters it accepts?

int strtotime ( string $time [, int $now = time() ] )

The function expects to be given a string containing an English date format and will try to parse that format into a Unix timestamp (the number of seconds since January 1 1970 00:00:00 UTC), relative to the timestamp given in now, or the current time if now is not supplied.

In particular we are interested in the first parameter, time.

time – A date/time string. Valid formats are explained in Date and Time Formats.

It is especially important to note what are the valid Date and Time Formats.

Here are the supported formats, but most interesting are those that are Relative.

Exactly these formats are very convenient in our case, because they give us the ability to work with human readable strings, and here are some examples from the documentation of strtotime().

echo strtotime("now"), "\n";
echo strtotime("10 September 2000"), "\n";
echo strtotime("+1 day"), "\n";
echo strtotime("+1 week"), "\n";
echo strtotime("+1 week 2 days 4 hours 2 seconds"), "\n";
echo strtotime("next Thursday"), "\n";
echo strtotime("last Monday"), "\n";

Thus, a valid string would be “1 month ago”.

// if current date is 2011-11-04, this will return 2011-10-04
echo date('Y-m-d', strtotime('1 month ago'))

Or “-1 month”:

// the same as the example above
echo date('Y-m-d', strtotime('-1 month'));

It’s interesting that “+1 -1 month” is also a valid string.

// 2011-10-04, if today's 2011-11-04
echo date('Y-m-d', strtotime('+1 -1 month'));

In fact strtotime() can do a lot more than most of the developers have ever imagined. Maybe its frequent use with string formatted dates (2010-01-13) makes it a bit unknown. Here are some interesting use cases.

// 1970-01-01, Calculations in braces are bad!
echo date('Y-m-d', strtotime('(60*60) minute'));

// 2 months into the future
echo date('Y-m-d', strtotime('-2 months ago'));

For instance, do you know how to get the date of the day before yesterday? Yes 2 days before today, but here’s yet another solution.

// 1 day before yesterday
echo date('Y-m-d', strtotime('yesterday -1 day'));

Another example is the fully human readable:

// get the first monday of the current month
echo date('Y-m-d', strtotime('first monday this month'));

The Solution of the Task

Finally, what is the solution of the original task? Well, just have to check whether a date is more or less than a month ago.

// a random date
$my_date = '2011-09-23';
	
// true if my_date is more than a month ago
(strtotime($my_date) < strtotime('1 month ago'))

Interfaces

Object interfaces allow you to create code which specifies which methods a class must implement, without having to define how these methods are handled.
Interfaces are defined using the interface keyword, in the same way as a standard class, but without any of the methods having their contents defined.
All methods declared in an interface must be public, this is the nature of an interface.

To implement an interface, the implements operator is used. All methods in the interface must be implemented within a class; failure to do so will result in a fatal error. Classes may implement more than one interface if desired by separating each interface with a comma.

Abstract Classes

PHP 5 introduces abstract classes and methods. Classes defined as abstract may not be instantiated, and any class that contains at least one abstract method must also be abstract. Methods defined as abstract simply declare the method’s signature – they cannot define the implementation.

When inheriting from an abstract class, all methods marked abstract in the parent’s class declaration must be defined by the child; additionally, these methods must be defined with the same (or a less restricted) visibility. For example, if the abstract method is defined as protected, the function implementation must be defined as either protected or public, but not private. Furthermore the signatures of the methods must match, i.e. the type hints and the number of required arguments must be the same. This also applies to constructors as of PHP 5.4. Before 5.4 constructor signatures could differ.

Some Cases

Now let’s do a quick experiment. According to the definition of interfaces we can define an interface and than an abstract class can implement this interface.

interface A
{
	public function a();
}

abstract class B implements A
{
	public function a()
	{
		return 10;
	}
}

However the abstract class cannot be instantiated so we need to extend it.

interface A
{
	public function a();
}

abstract class B implements A
{
	public function a()
	{
		return 10;
	}
}

class C extends B
{
	...
}

In the first place is completely OK to define an abstract class that does not contain any abstract method. TUsually if a class has one or more abstract methods (Methods defined as abstract simply declare the method’s signature – they cannot define the implementation, according to the docs), it must be defined as abstract. But we can also define an abstract class which has no abstract methods.

abstract class B
{
	public function a()
	{
		return 5;
	}
}

class C extends B
{
	public function a()
	{
		return 10;
	}
}

$c = new C();
echo $c->a();

OK then what would happen if we had an interface that is implemented by an abstract class, which in turn is extended by a given class. First of all if a class implements an interface, even if it is an abstract class, it must implement all methods defined in the interface. So if the predefined method in the abstract class is marked as abstract, it would result into a fatal error.

interface A
{
	public function a();
}

abstract class B implements A
{
	abstract public function a();
}

class C extends B
{
	public function a()
	{
		return 5;
	}
}

$c = new C();
echo $c->a();

This is perfectly logical, because in fact each method of the interface must be implemented by his children.

“All methods in the interface must be implemented within a class; failure to do so will result in a fatal error.”

Fine, but we can have abstract classes with no abstract methods, let’s try this:

interface A
{
	public function a();
}

abstract class B implements A
{
	public function a()
	{
		return 10;
	}
}

class C extends B
{
	public function a()
	{
		return 5;
	}
}

$c = new C();
echo $c->a();

This code is working and the result is the returned value from C::a(). However class B is abstract so let’s go even further. Let’s define an abstract method, different from the method B::a().

interface A
{
	public function a();
}

abstract class B implements A
{
	public function a()
	{
		return 10;
	}
	
	abstract public function b();
}

class C extends B
{
	public function a()
	{
		return 5;
	}
	
	public function b()
	{
		return "hello world!";
	}
}

$c = new C();
echo $c->a();
echo $c->b();

Now there’s a fatal error. Why?

$arr = array(1 => 10, 1 => 11);

What happens when the interpreter reaches this line of code? This is not a syntax error and it is completely valid. Very similar, but more interesting case is when we have an array of identical keys, where those identical keys are represented once as an integer and then as a string.

$arr = array(1 => 10, "1" => 11);

Now several questions arise. First of all, how many elements have this array? Two or one. This can be easily verified by checking what count() will return.

echo count($arr);

The correct answer is 1. This simply means, that there’s no difference between string keys and integer keys. What would happen if we had a “normal” array with different keys?

$arr = array(1 => 10, "2" => 11);
echo count($arr);

As expected this returns 2.

Next thing to check is what’s in the array after this initialization line.

$arr = array(1 => 10, "1" => 11);

Is there something in the first element $arr[0], or there’s something in the second element $arr[1]? What is the value of the single value?
As it appears the second element replaces the first one. We’ve seen that the array has only one value, but where’s that value? The only way to check this is to dump both elements:

var_dump($arr);

Here we can see that $arr[1] contains “11” and it is the only value, and $arr[0] is not set.