This is how I arrived to this message.

cd somewhere
git clone https://github.com/moodle/moodle.git m402
cd m402
git co MOODLE_402_STABLE
cp ../myoldmoodle/config.php ./

I changed in the copied config:

$CFG->dbname
$CFG->wwwroot
$CFG->dataroot

leaving untouched:

$CFG->debug = E_ALL | E_STRICT;
$CFG->debugdisplay = true;

I called moodle with a brower.

What I found was:

Deprecated: trim(): Passing null to parameter #1 ($string) of type string is deprecated in /Applications/MAMP/htdocs/m402/lib/pagelib.php on line 1408