PHP object properties

Question

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.

Answer 1

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.

Answer 2

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