The function that can be used to determine the type of an object property is `gettype()`. This function returns the type of the variable as a string. For example, if you want to determine the type of a property `someVar` of an object `$x`, you would use `gettype($x->someVar)`. The function `get_object_vars()` is used to get the properties of an object in an associative array, not to determine their types.