Caddy provides the http.handlers.vars module, but it isn't available for use in a Caddyfile. Sometimes setting a variable might be useful but not worth abandoning the simplicity of the Caddyfile configuration format.
Build with xcaddy:
xcaddy build --with github.com/edudobay/caddy-varsdirective
Rewriting the URL in a PHP/FastCGI app is not so obvious (see this thread for an explanation). In this example, we remove the /_api prefix, if present, and also remove trailing slashes.
route {
uri strip_prefix /_api
uri strip_suffix /
set_var resolved_uri {path}
php_fastcgi localhost:9000 {
env REQUEST_URI {http.vars.resolved_uri}
}
}