I thought the intention here was to skip just the current disabled one, so I'm surprised about the break instead of continue.

Maybe a better comment to clarify?

// The field is disabled, and also the others would always be null.

Should we dedicate another variable for this semantically different value, instead of replacing the original one?

It's always the same field... we only have one...
Would it be cleared if it was continue? It's more-or-less meaningless from the POV of performance (I'm not sure there's any non-test way to get into this loop with more then one object...)

we often re-purpose variables when we don't need the old value any more. Semantically they're the same, they are only different on a technical level.

This is equivalent of doing

$msg = 'foo';
$msg .= 'bar';

Maybe we can use $project = head($projects) to avoid to work inside a loop and just operate on the first element

no, the projects are different, the field instances are different, fields definitions are the same.

Each project instance needs its own field instance.

So in theory we should also reset $custom_fields if we find a NULL $field but in that case the $custom_fields is already empty so a break is enough since it always will happen in the first cycle. OK.

Again maybe a comment like

// The description is disabled for this Project, but also for any other.