I am working with Doctrine ORM and I often see recommendations to call toArray() on a collection before iterating over it when removing elements.

I would like to understand whether this is really necessary and why.

Context

I have a Dossier entity with a OneToMany relationship to Echeance:

#[ORM\OneToMany(mappedBy: 'dossier', targetEntity: Echeance::class, orphanRemoval: true)]
private Collection $echeances;

I want to remove all Echeance entities that do not have an active Reglement.

Code without toArray()

foreach ($dossier->getEcheances() as $echeance) {
    if (!$this->hasActiveReglement($echeance)) {
        $dossier->removeEcheance($echeance);
    }
}

I have seen advice saying that this pattern can be unsafe because the collection is modified while it is being iterated.

Alternative version with toArray()

foreach ($dossier->getEcheances()->toArray() as $echeance) {
    if (!$this->hasActiveReglement($echeance)) {
        $dossier->removeEcheance($echeance);
    }
}

Questions :

• Is it actually unsafe to remove elements from a Doctrine Collection while iterating over it?

• Can elements really be skipped in this situation, or is this just a precaution?

• Does this behavior come from PHP foreach itself, or from Doctrine’s PersistentCollection implementation?

• Is using toArray() the recommended best practice in this case?

I would like to understand what happens internally in this case.