Is swallowing phlegm forbidden? I hope you can clarify this with evidence.

Phlegm is pure, even if it is considered repulsive. Swallowing it for a non-fasting person, after it has become possible to expel it, is permissible but repulsive. As for the fasting person, swallowing it after it has reached the mouth invalidates their fast according to the majority of scholars. However, if they swallow it unintentionally or are unable to expel it, then there is no blame upon them and their fast is valid.