'silahkan' is still used in everyday speech, but the official spelling according to KBBI is 'silakan'.
silah, silahkan ? 1sila1/si·lah, si·lah·kan ? 1sila/
If you search for 'silahkan', there is no definition of the word.
You'll be directed to 'silakan'.
sila1/si·la/ v, silakan/si·la·kan/ v sudilah kiranya (kata perintah yang halus): - duduk;
The definition is listed under 'silakan'.