If Phil Cassidy were in GTA: San Andreas, it would be something like this:
After the events of GTA: Vice City, he decides to move to the West Coast of the USA, and opens his own exclusive Ammu-Nation, a name (which I invented): Phil's Ammu-Nation, because in addition to the weapons that appear in all other gun stores, he also sells 3 Melee Weapons (Baseball Bat, Knife and Chainsaw), 4 Heavy Weapons (RPG, Guided Missile Launcher, Minigun and Flamethrower) and 2 Throwable Weapons (Tear Gas Grenade and Molotov Cocktail) more than other Ammu-Nations.
 Phil would live in the Las Venturas desert, he drives the Perennial, the Walton, and the Freeway, and his favorite radio stations are K-DST and K-Rose, and just to "lie" about being an "ex-military man," he keeps 3 army vehicles in his garage: Patriot, Barracks, and Rhino.
After some events in San Andreas, a character in the story mode tells Carl "CJ" Johnson to find a certain Phil Cassidy, who claims to be an ex-military man and that he will help CJ with the supply of heavy weapons.
 After arriving at Phil's house, he, completely drunk, threatens CJ. CJ asks him to calm down, and CJ says that a friend of CJ's recommended him to help CJ if CJ also helps Phil. Then Phil lowers the gun and laughs, saying he wasn't going to kill him. He staggers, and CJ manages to grab him before he falls to the ground. Phil invites him into his house, and with no other option, CJ accepts the invitation and enters Phil's house (but deep down, CJ must be regretting listening to his friend and agreeing to help an alcoholic idiot).
Inside Phil Cassidy's new house, Carl "CJ" Johnson recounts the situations he's getting into and asks Phil for help if CJ also helps Phil. Phil agrees to help CJ, even saying that it's better to help others than to abandon them in the shit. Then he lies about a story he met a colleague in the army who had difficult situations, that only he can help, etc.  Phil said he built a new gun shop, but he doesn't have enough money to open it, so CJ says he'll buy it. Phil celebrates and falls over with joy (CJ is experiencing the most embarrassing moment he's ever had).
Outside the house, they both get in the car and head towards Phil's Ammu-Nation. There's a short dialogue between CJ and Phil, such as: Phil said he got into a nasty bar fight with one of the San Andreas Biker gang members, and said he lost his arm in a Great War in Nicaragua (when in reality, he lost his arm because he was too stupid, as he got too close to an explosion in Vice City, since he was never a soldier).  Upon arriving at the location, CJ compliments the facade and makes a purchase, becoming the owner of Phil's Ammu-Nation. The store has two floors: on the main floor, CJ buys weapons and has a Sprunk soda machine, and on the upper floor, there will be a shooting range and a vending machine for snacks and chocolate.
After purchasing Phil's Ammu-Nation, Phil Cassidy's missions will begin. I don't know what the missions would be, but I think they would involve arms trafficking, some disagreement with the army, or some conflict with the San Andreas Bikers. After completing all of Phil Cassidy's missions, CJ gets a 25% discount on all weapons, early access to heavy weapons, gets the keys to the 3 army vehicles, and the property will generate income, with a maximum profit of US$10,000.00.
 Obviously, after all the events of San Andreas, Phil Cassidy decides to return to the East Coast of the USA, but this time, to Liberty City, to have a new life, where the events of GTA: Liberty City Stories begin.
The image shows what Phil Cassidy would look like in GTA: San Andreas, kind of a transition between GTA: Vice City and Liberty City Stories.

Phil Cassidy in GTA San Andreas