fix(snippetz): keep every value of a repeated form field in php/guzzle examples - #10499

A URL-encoded form with repeated names loses data in the PHP Guzzle example. For tag=a&tag=b, the example previously sent only the last value. Guzzle also encodes arrays in form_params as indexed names, which would send tag[0]=a&tag[1]=b instead of the original fields.

Generate an encoded raw body when names repeat, preserving field names, order, empty values and URL encoding. Add the form Content-Type when absent and preserve an existing header. Forms without repeated names continue to use form_params.