With this code:
$a[1]=1;
$a[2]=& $a[1];
$b=$a;
$b[2]=7;
print_r($a);
I was expecting output to be 1 because $a is not assigned by reference to $b ($a = & $b) but it comes out to be 7. Why?
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3 Answers
You're copying the array by value, but the elements of the source array are references. These references are just shallow-copied to the destination.
php > var_dump($a);
array(2) {
[1]=>
&int(1)
[2]=>
&int(1)
}
php > $b=$a;
php > var_dump($b);
array(2) {
[1]=>
&int(1)
[2]=>
&int(1)
}
Here's an example copying the array by reference:
php > $c[1] = 1;
php > $c[2] =& $c[1];
php > var_dump($c);
array(2) {
[1]=>
&int(1)
[2]=>
&int(1)
}
php > $d =& $c;
php > var_dump($d);
array(2) {
[1]=>
&int(1)
[2]=>
&int(1)
}
php > $d = array(3,4,5);
php > var_dump($c);
array(3) {
[0]=>
int(3)
[1]=>
int(4)
[2]=>
int(5)
}
php > var_dump($d);
array(3) {
[0]=>
int(3)
[1]=>
int(4)
[2]=>
int(5)
}
As you can see, the array variable (not elements) itself is a reference, so modifying $d affects $c. Reassigning $b itself has no effect on $a.
Comments
That's how references work. What are you saying when you do $a[2]=& $a[1]; is that both elements of $a now hold the same variable. When you do $b = $a, $b and $a are different variables, but all 4 elements inside them point to the same variable! Try doing $b[3] = 7 and see it's not copied to $a - because $b and $a are different, but $b[2] and $a[2] are not!
References are a bit messy, sorry. Avoid using them unless you must.
3 Comments
bcosca
while the explanation is good, the advice to avoid the use of references is not.
StasM
@stillstanding: If you have to use them, definitely use them. But in many cases it's not necessary - e.g. I've seen php4 code using refs with objects, or passing arrays by refs "for speed", etc. This should not be done. There are legitimate refs uses of course.
This help me to understand what references are:
http://www.php.net/manual/en/features.gc.refcounting-basics.php
And debugging with xdebug of interesting cases helps too
For example you can "avoid" private properties:
class c{
private $a = 42;
function &a(){
return $this->a;
}
function print_a(){
echo $this->a;
}
}
$c = new c;
$d = &$c->a();
echo $d;
$d = 2;
$c->print_a();
or pass by reference to functions if you not declared it:
function f1(&$s) {
$s++;
}
function f2($s) {
$s--;
}
$s1 = 1;
$s2 = 3;
f1($s1);
f2(&$s2);
echo $s1.$s2;
also foreach can use pass-by-reference
$a = array(1,2,3); foreach ( $a as $key => &$value ) { $value = 1; } $value = 2;
