1.
If someone cracks up, they are under such a lot of emotional strain that they become mentally ill. (INFORMAL)
She would have cracked up if she hadn’t allowed herself some fun.
PHRASAL VERB: V P
2.
If you ~ or if someone or something cracks you up, you laugh a lot. (INFORMAL)
She told stories that cracked me up and I swore to write them down so you could enjoy them too...
We all just cracked up laughing.
PHRASAL VERB: V n P, V P