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. 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 */ ?>]]>