application/x-httpd-php

Not a type you send — a type you configure: it exists so Apache-style handlers can route .php files to the interpreter, and it survives in mime maps as a de facto entry with no registration behind it. If a response ever actually arrives labeled application/x-httpd-php, something has gone wrong — the server has stopped executing PHP and started serving its source, credentials and all, a classic misconfiguration after an upgrade. Executed PHP emits whatever the script emits, usually text/html.

Defined by PHP language reference. Registry facts from the IANA media-types registry via mime-db.