PHP $this variable

Question

I am reading some PHP code that I could not understand:

class foo {
  function select($p1, $dbh=null) {
    if ( is_null($dbh) )
        $dbh = $this->dbh         


        

Answer 1

1. With the code you've posted, you can't know what the values of $this->dbh is.
2. $dbh is a property of the current object. $this is use to access to the members of the current object.
3. Since this variable is defined outside of any function, is a variable that belongs to the class and not to a specific function. Because of this, $this->dbh can be used in any function inside the class.
4. PHP doesn't require to define every variable you use.