src/Model/Account.php
public static function isValidAccount($accountId)
    {
        // Validate that keypair passes checksum
        try {
            $keypair = Keypair::newFromPublicKey($accountId);
        } catch (\Exception $e) {
            return false;
        }

        return true;
    }
public static function isValidAccount($accountId)
    {
        // Validate that keypair passes checksum
        try {
            $keypair = Keypair::newFromPublicKey($accountId);
        } catch (\Exception $e) {
            return false;
        }

        return true;
    }
Returns true if the specified account ID (G....) passes basic validity checks

Note that this doesn't necessarily mean the account is funded or exists
on the network. To check that, use the Server::getAccount() method.

@param $accountId
@return bool