note 59820 added to language.references.spot

*** WARNING about OBJECTS TRICKY REFERENCES *** ----------------------------------------------- The use of references in the context of classes and objects, though well defined in the documentation, is somehow tricky, so one must be very careful when using objects. Let's examine the following two examples: <?php class y { public $d; } $A = new y; $A->d = 18; echo "Object \$A before operation:\n"; var_dump($A); $B = $A; // This is not an explicit (=&) reference assignment, // however, $A and $B refer to the same instance // though they are not equivalent names $C =& $A; // Explicit reference assignment, $A and $C refer to // the same instance and they have become equivalent // names of the same instance $B->d = 1234; echo "\nObject \$B after operation:\n"; var_dump($B); echo "\nObject \$A implicitly modified after operation:\n"; var_dump($A); echo "\nObject \$C implicitly modified after operation:\n"; var_dump($C); // Let's make $A refer to another instance $A = new y; $A->d = 25200; echo "\nObject \$B after \$A modification:\n"; var_dump($B); // $B doesn't change echo "\nObject \$A after \$A modification:\n"; var_dump($A); echo "\nObject \$C implicitly modified after \$A modification:\n"; var_dump($C); // $C changes as $A changes ?> Thus, note the difference between assignments $X = $Y and $X =& $Y. When $Y is anything but an object instance, the first assignment means that $X will hold an independent copy of $Y, and the second, means that $X and $Y will refer to the same thing, so they are tight together until either $X or $Y is forced to refer to another thing. However, when $Y happens to be an object instance, the semantic of $X = $Y changes and becomes only slightly different to that of $X =& $Y, since in both cases $X and $Y become references to the same object. See what this example outputs: Object $A before operation: object(y)#1 (1) { ["d"]=> int(18) } Object $B after operation: object(y)#1 (1) { ["d"]=> int(1234) } Object $A implicitly modified after operation: object(y)#1 (1) { ["d"]=> int(1234) } Object $C implicitly modified after operation: object(y)#1 (1) { ["d"]=> int(1234) } Object $B after $A modification: object(y)#1 (1) { ["d"]=> int(1234) } Object $A after $A modification: object(y)#2 (1) { ["d"]=> int(25200) } Object $C implicitly modified after $A modification: object(y)#2 (1) { ["d"]=> int(25200) } Let's review a SECOND EXAMPLE: <?php class yy { public $d; function yy($x) { $this->d = $x; } } function modify($v) { $v->d = 1225; } $A = new yy(3); var_dump($A); modify($A); var_dump($A); ?> Although, in general, a formal argument declared as $v in the function 'modify' shown above, implies that the actual argument $A, passed when calling the function, is not modified, this is not the case when $A is an object instance. See what the example code outputs when executed: object(yy)#3 (1) { ["d"]=> int(3) } object(yy)#3 (1) { ["d"]=> int(1225) } ---- Manual Page -- http://www.php.net/manual/en/language.references.spot.php Edit -- http://master.php.net/manage/user-notes.php?action=edit+59820 Delete: added to the manual -- http://master.php.net/manage/user-notes.php?action=delete+59820&report=yes&reason=added+to+the+manual Delete: bad code -- http://master.php.net/manage/user-notes.php?action=delete+59820&report=yes&reason=bad+code Delete: spam -- http://master.php.net/manage/user-notes.php?action=delete+59820&report=yes&reason=spam Delete: useless -- http://master.php.net/manage/user-notes.php?action=delete+59820&report=yes&reason=useless Delete: non-english -- http://master.php.net/manage/user-notes.php?action=delete+59820&report=yes&reason=non-english Delete: already in docs -- http://master.php.net/manage/user-notes.php?action=delete+59820&report=yes&reason=already+in+docs Delete: other reasons -- http://master.php.net/manage/user-notes.php?action=delete+59820&report=yes Reject -- http://master.php.net/manage/user-notes.php?action=reject+59820&report=yes Search -- http://master.php.net/manage/user-notes.php