Question

Lately I’ve been wondering if there’s a difference between initializing the variables that have a default value on the Constructor VS on the class definition.

Which one is better, taking into consideration the optimization:

class TestClass
{
 private $test_var = 'Default Value';
 function __construct()
 {
 }
}

class TestClass2
{
 private $test_var;
 function __construct()
 {
  $this->test_var = 'Default Value';
 }
}

Answer 1

The advantage of initializing properties outside of the constructor is that someone reading your code will immediatly know its a default value.

Inconvenient is you cannot use every kind of data this way — will not work with object instanciations, for instance, or with heredoc syntax, from what I recall.

I don’t think there is much of a difference when it comes to performance — anyway, there are probably lots of things that matter a lot more, in your application 😉

Still, purely for fun, using the Vulcan Logic Disassembler :

With the first example of code (temp-2.php) :

<?php
class TestClass {
    private $test_var = 'Default Value';
    function __construct() {
    }
}
$a = new TestClass();

You get these opcodes :

$ php -d extension=vld.so -d vld.active=1 temp-2.php
Branch analysis from position: 0
Return found
filename:       /home/squale/developpement/tests/temp/temp-2.php
function name:  (null)
number of ops:  11
compiled vars:  !0 = $a
line     #  op                           fetch          ext  return  operands
-------------------------------------------------------------------------------
   2     0  EXT_STMT
         1  NOP
   7     2  EXT_STMT
         3  ZEND_FETCH_CLASS                                 :1      'TestClass'
         4  EXT_FCALL_BEGIN
         5  NEW                                              $2      :1
         6  DO_FCALL_BY_NAME                              0
         7  EXT_FCALL_END
         8  ASSIGN                                                   !0, $2
         9  RETURN                                                   1
        10* ZEND_HANDLE_EXCEPTION

Class TestClass:
Function __construct:
Branch analysis from position: 0
Return found
filename:       /home/squale/developpement/tests/temp/temp-2.php
function name:  __construct
number of ops:  4
compiled vars:  none
line     #  op                           fetch          ext  return  operands
-------------------------------------------------------------------------------
   4     0  EXT_NOP
   5     1  EXT_STMT
         2  RETURN                                                   null
         3* ZEND_HANDLE_EXCEPTION

End of function __construct.

End of class TestClass.

While, with the second example of code (temp-3.php) :

<?php
class TestClass2 {
    private $test_var;
    function __construct() {
        $this->test_var = 'Default Value';
    }
}
$a = new TestClass2();

You get those opcodes :

$ php -d extension=vld.so -d vld.active=1 temp-3.php
Branch analysis from position: 0
Return found
filename:       /home/squale/developpement/tests/temp/temp-3.php
function name:  (null)
number of ops:  11
compiled vars:  !0 = $a
line     #  op                           fetch          ext  return  operands
-------------------------------------------------------------------------------
   2     0  EXT_STMT
         1  NOP
   8     2  EXT_STMT
         3  ZEND_FETCH_CLASS                                 :1      'TestClass2'
         4  EXT_FCALL_BEGIN
         5  NEW                                              $2      :1
         6  DO_FCALL_BY_NAME                              0
         7  EXT_FCALL_END
         8  ASSIGN                                                   !0, $2
   9     9  RETURN                                                   1
        10* ZEND_HANDLE_EXCEPTION

Class TestClass2:
Function __construct:
Branch analysis from position: 0
Return found
filename:       /home/squale/developpement/tests/temp/temp-3.php
function name:  __construct
number of ops:  7
compiled vars:  none
line     #  op                           fetch          ext  return  operands
-------------------------------------------------------------------------------
   4     0  EXT_NOP
   5     1  EXT_STMT
         2  ZEND_ASSIGN_OBJ                                          'test_var'
         3  ZEND_OP_DATA                                             'Default+Value'
   6     4  EXT_STMT
         5  RETURN                                                   null
         6* ZEND_HANDLE_EXCEPTION

End of function __construct.

End of class TestClass2.

So, I’m guessing there is a bit of a difference… But not that important ^^

Up to you to interpret the opcodes — but the funny thing is there is no trace of ‘Default Value‘ in the first dump… interesting, actually ^^

Seems VLD cannot (or just doesn’t) dump everything 🙁