What it does
Phalanx Commander isn't an official Magic keyword—you may be thinking of a custom or homebrew card, or possibly confusing it with something else. There's no such ability in the official rules.
You might be looking for a few real things: Phalanx Leader is a Theros creature whose heroic ability puts +1/+1 counters on all your creatures when you cast a spell targeting it. Phalanx as a flavor term shows up in card names, but it grants no mechanical rule by itself. And commander in EDH refers to your legendary general, governed by the "commander tax" (extra 2 generic mana each time you recast it from the command zone) and commander damage (21 from a single commander ends a player).
If you can tell me the exact card or where you saw "Phalanx Commander," I can explain precisely what it does and any rules gotchas tied to it.
Iconic cards with Phalanx Commander
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