References in PHP are a means to access the same variable content by different names. They are not like C pointers; for instance, you cannot perform pointer arithmetic using them, they are not actual memory addresses, and so on. See for more information. Instead, they are symbol table aliases. Note that in PHP, variable name and variable content are different, so the same content can have different names. The closest analogy is with Unix filenames and files - variable names are directory entries, while variable content is the file itself. References can be likened to hardlinking in Unix filesystem. There are three basic operations performed using references: assigning by reference, passing by reference, and returning by reference. This section will give an introduction to these operations, with links for further reading. In the first of these, PHP references allow you to make two variables refer to the same content. Meaning, when you do: ]]> it means that $a and $b point to the same content. $a and $b are completely equal here. $a is not pointing to $b or vice versa. $a and $b are pointing to the same place. If you assign, pass, or return an undefined variable by reference, it will get created. d); var_dump(property_exists($c, 'd')); // bool(true) ?> ]]> The same syntax can be used with functions that return references: ]]> Using the same syntax with a function that does not return by reference will give an error, as will using it with the result of the new operator. Although objects are passed around as pointers, these are not the same as references, as explained under Objects and references. If you assign a reference to a variable declared global inside a function, the reference will be visible only inside the function. You can avoid this by using the $GLOBALS array. ]]> Think about global $var; as a shortcut to $var =& $GLOBALS['var'];. Thus assigning another reference to $var only changes the local variable's reference. If you assign a value to a variable with references in a &foreach; statement, the references are modified too. ]]> While not being strictly an assignment by reference, expressions created with the language construct array() can also behave as such by prefixing & to the array element to add. Example: ]]> Note, however, that references inside arrays are potentially dangerous. Doing a normal (not by reference) assignment with a reference on the right side does not turn the left side into a reference, but references inside arrays are preserved in these normal assignments. This also applies to function calls where the array is passed by value. Example: ]]> In other words, the reference behavior of arrays is defined in an element-by-element basis; the reference behavior of individual elements is dissociated from the reference status of the array container. The second thing references do is to pass variables by reference. This is done by making a local variable in a function and a variable in the calling scope referencing the same content. Example: ]]> will make $a to be 6. This happens because in the function foo the variable $var refers to the same content as $a. For more information on this, read the passing by reference section. The third thing references can do is return by reference. As said before, references are not pointers. That means, the following construct won't do what you expect: ]]> What happens is that $var in foo will be bound with $bar in the caller, but then re-bound with $GLOBALS["baz"]. There's no way to bind $bar in the calling scope to something else using the reference mechanism, since $bar is not available in the function foo (it is represented by $var, but $var has only variable contents and not name-to-value binding in the calling symbol table). You can use returning references to reference variables selected by the function. You can pass a variable by reference to a function so the function can modify the variable. The syntax is as follows: ]]> There is no reference sign on a function call - only on function definitions. Function definitions alone are enough to correctly pass the argument by reference. The following things can be passed by reference: Variables, i.e. foo($a) References returned from functions, i.e.: ]]> See more about returning by reference. No other expressions should be passed by reference, as the result is undefined. For example, the following examples of passing by reference are invalid: ]]> 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: 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. 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 $myValue. If you try to return a reference from a function with the syntax: return ($this->value); this will not work as you are attempting to return the result of an expression, and not a variable, by reference. You can only return variables by reference from a function - nothing else. To use the returned reference, you must use reference assignment: ]]> To pass the returned reference to another function expecting a reference you can use this syntax: ]]> Note that array_push(&collector(), 'foo'); will not work, it results in a fatal error. When you unset the reference, you just break the binding between variable name and variable content. This does not mean that variable content will be destroyed. For example: ]]> won't unset $b, just $a. Again, it might be useful to think about this as analogous to the Unix unlink call. Many syntax constructs in PHP are implemented via referencing mechanisms, so everything mentioned herein about reference binding also applies to these constructs. Some constructs, like passing and returning by reference, are mentioned above. Other constructs that use references are: When you declare a variable as global $var you are in fact creating reference to a global variable. That means, this is the same as: ]]> This also means that unsetting $var won't unset the global variable.