{check "..."} asks the database whether a row exists and answers TRUE or FALSE: on a hit the content renders, otherwise the @else@ branch does (or nothing). It is the template form of db ( "CHECK ..." ), and CHECK has a form of its own: a bare table followed by a WHERE clause - {check "orders where status='open'"}, never {check "* from orders ..."}.

Nothing of the row is read, so no field of it is available in the content - use {record} when the row itself is wanted. For a test on a value that is already in a variable, {if} is the tag.

As with the other database tags, a request value belongs in a placeholder in PHP: $taken = db ( "CHECK users WHERE email='{0}'", [ $email ] ); and then {if $taken}.