I headcannon that Phineas has meltdowns. He has very BIG feelings and sometimes they're overwhelming. When he was really little he would scream and cry. He thought maybe he'd grow out of it but it still happened when he was 10. Up to that point he did have them less and less but it still happens. When he has a meltdown he'll scream at people and loose speech only talking with one word sentences. At 3-4 he would kick his feet but not so much later.
In my Doof Dad AU in the Second Dimension movie Phineas gets overwhelmed by everything and has a meltdown. Learning his Dad is an Evil Scientist made him panic and something in his child brain snapped in reaction to it. Ferb tried putting a hand on his shoulder because it usually helped but this time Phineas was so overstimulated that he slapped his arm away. After he was able to calm down he apologized to Ferb and felt so guilty about hitting him. Ferb understands that this is just a very very stressful day for him.
He never got punished for having meltdowns his Mom usually just waited it out. When he was 3-4 his Mom taught him how to do deep breaths when he felt his emotions get too big. Instead of kicking he learned to just tense up his body. He will sometimes pull at his hair. His family is generally pretty understanding but he still feels embarrassed that he gets that way. He's usually not able to speak a few minutes after the meltdown and usually will isolate himself and only let Ferb sit quietly next to him
Phineas is generally a very happy person but his ADHD makes it hard to regulate his emotions sometimes. Most people wouldn't even suspect he has anger issues he's just learned how to cope with it.
