PHP OOP: why does self::$$property[$key] not work? - php

Question

Is there a way to implement the following PHP class without the foreach loop (and otherwise un-changed)?
<?php
class foo {
private static $_thing = array(
'kee' => 'valyu'
);
public static function fetch($property, $key = '') {
if (property_exists('foo', $property)) {
if ($key == '') return self::$$property;
else {
# i cannot seem to do self::$$property[$key].
foreach (self::$$property as $_key => $_value) {
if ($_key == $key) return $_value;
}
}
}
return false;
}
}
var_dump(foo::fetch('bad')); # null
var_dump(foo::fetch('_thing')); # array(1) { ... }
var_dump(foo::fetch('_thing', 'bad')); # null
var_dump(foo::fetch('_thing', 'kee')); # string(5) "valyu"
?>
self::$$property[$key] gives me "access to undeclared static property: foo::$x" (x being the first character in the string value of $key).

Answer 1

It's simply a quirk of the php parser, there seem to be no way to tell the parse with syntax that you want the variable-variable resolved first and then use the [] on it's result instead of the $property[$key] first, in one line.
However if you break it into two it will work fine:
class foo {
private static $_thing = array(
'kee' => 'valyu'
);
public static function fetch($property, $key = '') {
if (property_exists('foo', $property)) {
if ($key == '') {
return self::$$property;
} else {
$prop = self::$$property; // move the result to temporary variable, first part of $$property[$key]
if (array_key_exists($key, $prop)) { //isset will return false if the key is in fact set to null
return $prop[$key]; // use it, second part of $$property[$key]
}
}
}
return null;
}
}
var_dump(foo::fetch('bad')); // null
var_dump(foo::fetch('_thing')); // array(1) { ... }
var_dump(foo::fetch('_thing', 'bad')); // null
var_dump(foo::fetch('_thing', 'kee')); // string(5) "valyu"
Update:
As Dan kindly pointed out, the {} syntax can be used to disambiguate the intent for the parser like this:
public static function fetch($property, $key = '') {
if (property_exists('foo', $property)) {
if ($key == '') {
return self::$$property;
} else if (array_key_exists($key, self::${$property})) {
return self::${$property}[$key];
}
}
return null;
}
I've also changed the last return from false to null so it matches your example's comments.