The bad thing about trying the ftp_chdir is not the need to suppress the errors. That's ok, as long as you have a legitimate reason to expect an error. It's rather the side affect of changing the directory.
If I take that direction, I'd try the ftp_size instead, as it does not have any side effects. It should fail for directories and succeed for files.
The ideal solution is to use the MLSD FTP command that returns a reliable machine-readable directory listing. But PHP supports that only since 7.2 with its ftp_mlsd function. Check the "type" entry for dir value.
Or, there's an implementation of the MLSD in user comments of the ftp_rawlist command:
https://www.php.net/manual/en/function.ftp-rawlist.php#101071
First check if your FTP server supports MLSD before taking this approach, as not all FTP servers do (particularly IIS and vsftpd don't).
Or, if you are connecting to one specific server, so you know its format of directory listing, you can use the ftp_rawlist, and parse its output to determine, if the entry is file or folder.
Typical listing on a *nix server is like:
drwxr-x--- 3 vincent vincent 4096 Jul 12 12:16 public_ftp
drwxr-x--- 15 vincent vincent 4096 Nov 3 21:31 public_html
-rwxrwxrwx 1 vincent vincent 11 Jul 12 12:16 file.txt
It's the leading d that tells you, if the entry is a directory or not.
You may be lucky and in your specific case, you can tell a file from a directory by a file name (i.e. all your files have an extension, while subdirectories do not).