Your script possibly relies on a session side-effect which existed until PHP 4.2.3. Please be advised that the session extension does not consider global variables as a source of data, unless register_globals is enabled. You can disable this functionality and this warning by setting session.bug_compat_42 or session.bug_compat_warn to off, respectively.

[full error message:
"Your script possibly relies on a session side-effect which existed
until PHP 4.2.3. Please be advised that the session extension does
not consider global variables as a source of data, unless
register_globals is enabled. You can disable this functionality and
this warning by setting session.bug_compat_42 or
session.bug_compat_warn to off, respectively."
]

The following short script causes the bug_compat_42 warning to appear.

<?php
session_start();
$_SESSION['userId'] = NULL;
$userId = “foo”;
?>

It took me an hour to find out this – so I post it here to avoid
that more people need such a long time.

Conclusion and test results:

You’ll get this warning if $_SESSION['userId'] contains NULL and you assign
anything (except NULL) to the global variable $var.

The warning will _not_ appear:
- if $_SESSION['userId'] contains anything else – or -
- if you don’t use a global variable named $userId