It seems the RFC[1] contradicts itself:

| It is implemented by using the result of the existing internal
| compare_function that underlies the other comparison operators.

but:

| // only values are compared
| $a = (object) ["a" => "b"]; 
| $b = (object) ["b" => "b"]; 
| echo $a <=> $b; // 0

However:

    var_dump($a < $b); // => bool(false)
    var_dump($b < $a); // => bool(false)
    
And currently:

    echo $a <=> $b; // => 1
    echo $b <=> $a; // => 1
    
Apparently, there is a problem with comparing unordered values with
the spaceship operator.
    
[1] <https://wiki.php.net/rfc/combined-comparison-operator>

This ends up using `zend_hash_compare`; see http://lxr.php.net/xref/PHP_TRUNK/Zend/zend_hash.c#zend_hash_compare

In this case it cannot find key "a" in $b so it returns 1.

Comparing such objects is not a meaningful operation, in general, so the result is not well-defined. But I don't see where the discrepancy is.

This is just a mistake in the RFC, not in the implementation, unless this doesn't match the behaviour of <, <=, ==, >= or >.

Found something else today

http://3v4l.org/mO48q

echo [1, 2, 3] <=> []; // should be 1 but it gives 3

The fix for this bug has been committed.

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/.

 For Windows:

http://windows.php.net/snapshots/
 
Thank you for the report, and for helping us make PHP better.

fixed*