Note: $a and $b are completely equal here, that's not $a is pointing to $b or vice versa, that's $a and $b pointing to the same place.

Note: If array with references is copied, its values are not dereferenced. This is valid also for arrays passed by value to functions.

Note: If you assign, pass or return an undefined variable by reference, it will get created.

Example 21-1. Using references with undefined variables <?php function foo(&$var) { } foo($a); // $a is "created" and assigned to null $b = array(); foo($b['b']); var_dump(array_key_exists('b', $b)); // bool(true) $c = new StdClass; foo($c->d); var_dump(property_exists($c, 'd')); // bool(true) ?>

Note: Not using the & operator causes a copy of the object to be made. If you use $this in the class it will operate on the current instance of the class. The assignment without & will copy the instance (i.e. the object) and $this will operate on the copy, which is not always what is desired. Usually you want to have a single instance to work with, due to performance and memory consumption issues.

While you can use the @ operator to mute any errors in the constructor when using it as @new, this does not work when using the &new statement. This is a limitation of the Zend Engine and will therefore result in a parser error.

Note: If you assign a value to a variable with references in a foreach statement, the references are modified too.

Example 21-3. References and foreach statement <?php $ref = 0; $row =& $ref; foreach (array(1, 2, 3) as $row) {     // do something } echo $ref; // 3 - last element of the iterated array ?>