file upload php $_FILES undefined index error

The error "Undefined index: (name of the file input)" in PHP usually occurs when the form that is used to upload the file is not properly configured. This can happen if the form's "enctype" attribute is not set to "multipart/form-data" or if the form does not have a "name" attribute for the file input field. Make sure your form's enctype is set correctly and the file input field has a name attribute.

Additionally, check if your file input field's name is correct and match with the $_FILES array index. You might also want to check if your server is configured to handle file uploads, and if the PHP settings (such as upload_max_filesize) allow the file size you are trying to upload.