I'm new to OOP in PHP and I find the difference between the following two expressions difficult to understand.
$object->$foo;
$object->foo;
Maybe it's my fault, but I could not find the relevant part in the manual.
3
2 Answers
The first call $obj->$foo is using a so called variable variable. Check this:
class A {
public $foo = 1;
}
$a = new A();
$foo = 'foo';
// now you can use both
echo $a->$foo;
echo $a->foo;
Follow the manual about variable variables
2 Comments
Shankar Narayana Damodaran
Nice one hek, Perfect. +1
hek2mgl
Thanks :) I must admit that my first, sunday morning, look also told me that the first line is wrong ;)
Well, in order to fully understand the somewhat odd-looking $object->$foo, you should understand two things about PHP:
Variable names
Most of the time variables in PHP are quite straight-forward. They begin with a $ sign, have one [a-zA-Z_] character, and then any amount of [a-z-A-Z0-9_] characters. Examples include:
$var = 'Abcdef';
$_GET = [];
$a1 = 123;
// And so on...
Now, PHP variables can actually be named pretty much anything, as long as the name is, or can be cast to, a scalar type. The way you name a variable with anything is to use curly braces ({}), like this:
${null} = 'It works'; echo ${null};
${false} = 'It works'; echo ${false};
${'!'} = 'It works'; echo ${'!'};
// Slightly weirder...
${(int)trim(' 5 ')} = 'It works'; echo ${5};
${implode(['a','b','c'])} = 'It works'; echo $abc;
Important: Just because you can do this does not mean you should, however. It is mostly just an oddity of PHP that you can do this.
Variable variables
A somewhat convoluted explanation: A variable variable is a variable that is accessed using a variable name.
A much easier way to understand variable variables is to use what we just learning about variable names in PHP. Take this example:
${"abc"} = 'Abc...';
echo $abc;
We create a variable using the string, "abc", which can also be accessed using $abc.
Now, there is no reason (or rule) that says it has to be a string.... it can also be a variable:
$abc = 'Abc...';
$varName = 'abc';
echo ${$varName}; // echo $abc
That is basically a variable variable. "Real" variable variables just do not use the curly braces:
$abc = 'Abc...';
$varName = 'abc';
echo $$varName; // echo $abc
As for the question
In the question the $object->$foo thing is basically just an "object variable variable", if you like
$object = new stdClass;
$object->abc = 'The alphabet!';
$foo = 'abc';
echo $object->$foo;
echo $object->{$foo}; // The same
echo $object->{'abc'}; // The same
Object variable variables can be somewhat useful, but they are rarely necessary. Using an associative array is usually a better choice.
1 Comment
martjahu
Thanks for extending the view. Very useful.
ClassName::$foofor accessing static properties and$object->foofor accessing object members$symbol.