In php when initializing a class how would one pass a variable to that class to be used in its functions?

Question

I want to call a class and pass a value to it so it can be used inside that class

The concept is called "constructor".

As the other answers point out, you should use the unified constructor syntax (__construct()) as of PHP 5. Here is an example of how this looks like:

class Foo {
    function __construct($init_parameter) {
        $this->some_parameter = $init_parameter;
    }
}

// in code:

$foo = new Foo("some init value");

---

Notice - There are so-called old style constructors that you might run into in legacy code. They look like this:

class Foo {
    function Foo($init_parameter) {
        $this->some_parameter = $init_parameter;
    }
}

This form is officially deprecated as of PHP 7 and you should no longer use it for new code.

In new versions of PHP (5 and up), the function __constuct is called whenever you use "new {Object}", so if you want to pass data into the object, add parameters to the construct function and then call

$obj = new Object($some, $parameters);

class Object {
    function __construct($one, $two) {}
}

Named constructors are being phased out of PHP in favor of the __construct method.

class SomeClass
{
    public $someVar;
    public $otherVar;

    public function __construct()
    {
        $arguments = func_get_args();

        if(!empty($arguments))
            foreach($arguments[0] as $key => $property)
                if(property_exists($this, $key))
                    $this->{$key} = $property;
    }
}

$someClass = new SomeClass(array('someVar' => 'blah', 'otherVar' => 'blahblah'));
print $someClass->someVar;

This means less maintenance in the long run.

Order of passed variables is not important anymore, (no more writing defaults like 'null': someClass(null, null, true, false))

Adding a new variable is less hassle (don't have to write the assignment in the constructor)

When you look at the instantiation of the class you'll know immediately what the passed in variables relate to:

Person(null, null, true, false) 

vs

Person(array('isFat' => true, 'canRunFast' => false)) 

This is how I do mine

class MyClass {

       public variable;  //just declaring my variables first (becomes 5)
       public variable2; //the constructor will assign values to these(becomes 6)

       function __construct($x, $y) {
            $this->variable  = $x;
            $this->variable2 = $y;
       }

       function add() {
            $sum = $this->variable + $this->variable2
            return $sum;
       }

} //end of MyClass class

Create an instance and then call the function add

$myob = new MyClass(5, 6); //pass value to the construct function
echo $myob->add();

11 will be written to the page not a very useful example because you would prefer to pass value to add when you called it but this illustrates the point.

You can do this like that:

class SomeClass
{
   var $someVar;
   function SomeClass($yourValue)
   {
       $this->someVar = $yourValue;
   }

   function SomeFunction()
   {
       return 2 * $this->someVar;
   }
}

or you can use __construct instead of SomeClass for constructor in php5.