Resolving PHP warnings when a file field is used in an email notification
Symptom: PHP warnings appear in the error log (or on-screen, if WP_DEBUG_DISPLAY is on) when a form submission triggers an email notification that references an ACF file field — typically Trying to access array offset on value of type bool/int/string, or Undefined index: url / Undefined index: filename.
Cause: Advanced Forms’ email-notification handling expects the file field to return an array (the full attachment object — id, url, filename, etc.). When the field’s Return Format is set to File URL or File ID, the merge-tag logic tries to read array keys off a string or integer, and PHP warns.
Fix: Update Advanced Forms to the latest version — the file-field handling has been hardened to cope with all three return formats. If you can’t update right away, change the file field’s Return Format to File Array under the field group’s edit screen and re-save the field group.
If you want to keep the field’s return format as-is for other reasons (e.g. other code on the site already consumes the URL or ID directly), the warnings will go away once you’re on a recent version of Advanced Forms.
See also
Section titled “See also”- How to add attachments to a notification email — covers attaching the uploaded file itself to the outgoing email.
- Customising email notifications — the merge-tag and template basics for notification emails.