instantiate a class from a variable in PHP?

Put the classname into a variable first:

$classname=$var.'Class';

$bar=new $classname("xyz");

This is often the sort of thing you'll see wrapped up in a Factory pattern.

See Namespaces and dynamic language features for further details.

class Test {
    public function yo() {
        return 'yoes';
    }
}

$var = 'Test';

$obj = new $var();
echo $obj->yo(); //yoes

How to pass dynamic constructor parameters too

If you want to pass dynamic constructor parameters to the class, you can use this code:

$reflectionClass = new ReflectionClass($className);

$module = $reflectionClass->newInstanceArgs($arrayOfConstructorParameters);

More information on dynamic classes and parameters

PHP >= 5.6

As of PHP 5.6 you can simplify this even more by using Argument Unpacking:

// The "..." is part of the language and indicates an argument array to unpack.
$module = new $className(...$arrayOfConstructorParameters);

Thanks to DisgruntledGoat for pointing that out.

If You Use Namespaces

In my own findings, I think it's good to mention that you (as far as I can tell) must declare the full namespace path of a class.

MyClass.php

namespace com\company\lib;
class MyClass {
}

index.php

namespace com\company\lib;

//Works fine
$i = new MyClass();

$cname = 'MyClass';

//Errors
//$i = new $cname;

//Works fine
$cname = "com\\company\\lib\\".$cname;
$i = new $cname;