What does the PHP syntax $var1->$var2 mean?

Question

What is the explanation for the following syntax?

$var1->$var2 // Note the second $

Answer 1

It means dynamically query a property in an object.

class A {
  public $a;
}

// static property access
$ob = new A;
$ob->a = 123;
print_r($ob);

// dynamic property access
$prop = 'a';
$ob->$prop = 345; // effectively $ob->a = 345;
print_r($ob);

so $var1 is an instance of some object, -> means access to a member of that object and $var2 contains the name of a property.