How to programmatically validate Tether (TRC20) Wallet Addresses in PHP?

trc20 address features:

An encoded Mainnet address begins with T and is 34 bytes in length.

<?php
    function isTrc20($address){
       return substr($address,0,1)=="T" and strlen($address)==34;
    }
    if(isTrc20("TC74QG8tbtixG5Raa4fEifywgjrFs45fNz"))
       echo "yes";
    else
        echo "no";

information source : https://medium.com/tron-foundation/tron-developer-guide-account-2446633a750

validating the address on your own is not preferred you should use an API like Tron protocol not only to check if the address is valid also to check if it's not a spam

you need to check the documentation
tronprotocol

also check how to make http request
here

also Shielded TRC-20

check this website as a reference

Regex for matching Tron address: T[A-Za-z1-9]{33}