In PHP 4, there was no concept of object properity protection. i.e. any chunk of code could modify a member variable's value.

PHP 5 now provides ways to prevent that from happening by introducing private, public, and protected member variables and methods.

<?php
    class foo {
        /* Only accessable within an instance of foo */
        private $foovar;
        
        /* Accessable from foo, or a subclass of foo */
        protected $foofamilyvar;
        
        /* Accessable from anywhere */
    
        public $a_var;

        public function mypublicfunc() { /* ... */ }

        private function myprivatefunc() { /* ... */ }

        protected function myprotectedfunc() { /* ... */ }
    }

    class bar extends foo {

        public function modify() {
    
            $this->foofamilyvar = true;
        }
    }

    $var_one = new foo();
    $var_two = new bar();

    $var_one->a_var = 10; /* acceptable, public variable */
    $var_one->foofamilyvar = true; /* error, protected var */
    $var_one->foovar = "Hi!" /* error, private var */

    $var_two->modify(); /* modifies the variable */

?>