Question

Take the following code from CodeIgniter’s show_error function:

$_error =& load_class('Exceptions', 'core');

The documentation for the load_class function says it acts as a singleton. The function basically takes the given parameters and searches for a class in the appropriate path. It then includes the file if it exists. The function is declared as:

function &load_class(...)

Why does it have the & and what is its purpose? Is $_error declared as such as a result of defining the function like that?

Answer 1

The php documentation seems to explain why you have to uses =& even though the function is marked to return a refrence function &load_class

Returning References

Returning by reference is useful when you want to use a function to
find to which variable a reference should be bound. Do not use
return-by-reference to increase performance. The engine will
automatically optimize this on its own. Only return references when
you have a valid technical reason to do so. To return references, use
this syntax:

 <?php class foo {
     public $value = 42;

     public function &getValue() {
         return $this->value;
     } 
  }



$obj = new foo; 
$myValue = &$obj->getValue(); // $myValue is a reference to $obj->value, which is 42.
$obj->value = 2; 
echo $myValue;
// prints the new value of $obj->value, i.e. 2. ?> 

In this example,

the property of the object returned by the getValue function would be
set, not the copy, as it would be without using reference syntax.
Note: Unlike parameter passing, here you have to use & in both places
– to indicate that you want to return by reference, not a copy, and to indicate that reference binding, rather than usual assignment, should
be done for $myValue.

If you are asking what references in general are the documentation explains.