The libcurl library documetation says it:

Using CURLOPT_POSTFIELDS implies setting CURLOPT_POST to 1.

See https://curl.se/libcurl/c/CURLOPT_POSTFIELDS.html

PHP uses libcurl library

PHP supports libcurl, a library created by Daniel Stenberg,

See https://www.php.net/manual/en/intro.curl.php