[L, QSA] Bandera
#Before: https://example.com/index.php?user=robert
RewriteEngine On
RewriteRule ^user/([^/]+)?$ index.php?user=$1 [L,QSA]
#After: https://example.com/user/robert?query=value
#You can also use other query too.
#Like: robert?query=value&query2=value2&query3=value3
Mr. Codex