1. Google "php steamid to steamid64"
2. Find thread Convert STEAMID to Steam Community ID
, where it links directly to:
3. Post #336 from 2014
Bingo! We've got a winner.
However, I would suggest you to use SteamID.php
That library literally accepts any kind of Steam ID format, and can output literally any (of the other) formats as well, and ... it is regularly being updated when and if something changes / breaks / new things are added. Stuff like that one ( ... and keeping it up-to-date) could actually simply your (Steam) life a lot. ;)