In addition to the above description I would like to add that the sub-classes CAN access each other's protected variables, if the accessing is done in a shared parent class.

While looking for a workaround I found that the result of the test on a protected method instead of a protected property is different (successful):

Workaround:
-----------
Add a protected method in the same class in which the protected property is defined that returns the value of the property. Then call the method in place of attempting to access the property.

Here is simplified reproduce code:

<?php

class A {
    protected $value;

    public function __construct($val) {
        $this->value = $val;
    }

    public function copyValue($obj) {
        $this->value = $obj->value;
    }
}

class B extends A {
    public function copyValue($obj) {
        $this->value = $obj->value;
    }
}
class C extends A {}

$B = new B("Value from B");
$C = new C("Value from C");

$B->copyValue($C); // fatal

var_dump($B);

?>

I'm not sure that this is a bug. Class B's copyValue() is trying to access a protected member of Class C, which is not in B's chain of inheritance.

The property is not being redeclared in C, though. It is still a property of A, structure-wise. A method declared and called in the same way as the property does not cause any error. Here's a simple non-code example:

protected method() is declared in A.
protected $property is declared in A.
B and C both extend A.
Method in B tries to access $C->property [FATAL ERROR]
Method in B tries to access $C->method() [No error]

One could even make method() return $this->property which would not cause an error either. It would be expected that both the property and method would have the same visibility when declared in the same class.

Odd. Here is a complete example.
<?php

class A {
    protected $value;
    public function __construct($val) {
        $this->value = $val;
    }
    public function copyValue($obj) {
        $this->value = $obj->value;
    }
    protected function getValue() {
        return $this->value;
    }
}

class B extends A {
    public function copyValue($obj) {
        $this->value = $obj->getValue(); // this works
        $this->value = $obj->value; // this is fatal
    }
}
class C extends A {}

$B = new B("Value from B");
$C = new C("Value from C");
$B->copyValue($C);

?>

Strange that the method call works but the property access doesn't. Both are protected.

Probably a missing check if the property was declared in a common parent class?

The problem is that right now struct property_info does not know where the property was declared. Thus what the engine sees is B accessing some protected property in C. The fix seems however quite easy. I'll give it a try.

Fix requies API change so it is not doable in 5.1.

Fixed in HEAD so far.

This bug has been fixed in CVS.

Snapshots of the sources are packaged every three hours; this change
will be in the next snapshot. You can grab the snapshot at
http://snaps.php.net/.
 
Thank you for the report, and for helping us make PHP better.

Fixed in 5.2, cannot fix in 5.0/5.1