Does swallowing phlegm or mucus break one's fast?

If a fasting person or a worshiper intentionally swallows phlegm, some scholars hold that their fast and prayer become invalid, while others do not hold this view, and this is the more preponderant opinion. This is because getting rid of them at all times involves hardship and difficulty. However, if one swallows it out of forgetfulness or involuntarily, there is no disagreement that it does not cause harm.