Can't get array of functions for some standard PHP extensions

I'm trying to get an array of function names for extensions Refletion, mysqlnd, xmlreader, Phar, pdo_mysql, xsl using standard get_extension_funcs() but it returns false instead of array of names. This is strange as it returns names for any other module. For example, it works for xmlwriter. But it does not work for xmlreader.

I have PHP 8.0.2. What am I doing wrong? Sample code:

$ php -r "var_dump(get_extension_funcs('xmlreader'));"
bool(false)

$ php -r "var_dump(get_extension_funcs('xmlwriter'));"
array(42) {
  [0]=>
  string(18) "xmlwriter_open_uri"
  [1]=>
  string(21) "xmlwriter_open_memory"
  [2]=>
  string(20) "xmlwriter_set_indent"
  [3]=>
  string(27) "xmlwriter_set_indent_string"
  [4]=>
  string(23) "xmlwriter_start_comment"
  [5]=>
  string(21) "xmlwriter_end_comment"
  [6]=>
  string(25) "xmlwriter_start_attribute"
  [7]=>
  string(23) "xmlwriter_end_attribute"
  [8]=>
  string(25) "xmlwriter_write_attribute"
  [9]=>
  string(28) "xmlwriter_start_attribute_ns"
  [10]=>
  string(28) "xmlwriter_write_attribute_ns"
  [11]=>
  string(23) "xmlwriter_start_element"
  [12]=>
  string(21) "xmlwriter_end_element"
  [13]=>
  string(26) "xmlwriter_full_end_element"
  [14]=>
  string(26) "xmlwriter_start_element_ns"
  [15]=>
  string(23) "xmlwriter_write_element"
  [16]=>
  string(26) "xmlwriter_write_element_ns"
  [17]=>
  string(18) "xmlwriter_start_pi"
  [18]=>
  string(16) "xmlwriter_end_pi"
  [19]=>
  string(18) "xmlwriter_write_pi"
  [20]=>
  string(21) "xmlwriter_start_cdata"
  [21]=>
  string(19) "xmlwriter_end_cdata"
  [22]=>
  string(21) "xmlwriter_write_cdata"
  [23]=>
  string(14) "xmlwriter_text"
  [24]=>
  string(19) "xmlwriter_write_raw"
  [25]=>
  string(24) "xmlwriter_start_document"
  [26]=>
  string(22) "xmlwriter_end_document"
  [27]=>
  string(23) "xmlwriter_write_comment"
  [28]=>
  string(19) "xmlwriter_start_dtd"
  [29]=>
  string(17) "xmlwriter_end_dtd"
  [30]=>
  string(19) "xmlwriter_write_dtd"
  [31]=>
  string(27) "xmlwriter_start_dtd_element"
  [32]=>
  string(25) "xmlwriter_end_dtd_element"
  [33]=>
  string(27) "xmlwriter_write_dtd_element"
  [34]=>
  string(27) "xmlwriter_start_dtd_attlist"
  [35]=>
  string(25) "xmlwriter_end_dtd_attlist"
  [36]=>
  string(27) "xmlwriter_write_dtd_attlist"
  [37]=>
  string(26) "xmlwriter_start_dtd_entity"
  [38]=>
  string(24) "xmlwriter_end_dtd_entity"
  [39]=>
  string(26) "xmlwriter_write_dtd_entity"
  [40]=>
  string(23) "xmlwriter_output_memory"
  [41]=>
  string(15) "xmlwriter_flush"
}

Even if I assume that some extensions does not have any functions, there are some extensions which SHOULD have functions, eg. mysqlnd should have mysqli_fetch_all() as noted in official docs.

Also, I assure that all those extensions are loaded and working fine.

$ php -r "var_dump(extension_loaded('mysqlnd'));"
bool(true)

$ php -r "var_dump(get_extension_funcs('mysqlnd'));"
bool(false)