Why can't you find information about php-rf < function name > on some php functions?

when I use the php-rf is_array command under the command line, I can print out the relevant is_array information, but when I use the same way to get the relevant information about the isset function and the empty function, it shows that the isset function does not exist and the empty function does not exist;

 C:\Users\longxiangde>php --rf is_array
Function [ <internal:standard> function is_array ] {

  - Parameters [1] {
    Parameter -sharp0 [ <required> $var ]
  }
}

C:\Users\longxiangde>php --rf empty
Exception: Function empty() does not exist


C:\Users\longxiangde>php --rf isset
Exception: Function isset() does not exist
    

excuse me, what kind of functions can php-rf print out? Thank you!

Php
Mar.22,2021

empty , isset is not a function


isset, empty are language structures

/Zend/zend_language_parser.h
/* Tokens.  */
-sharpdefine T_EMPTY 359
-sharpdefine T_ISSET 358
/Zend/zend_language_parser.y
T_EMPTY '(' expr ')' { $$ = zend_ast_create(ZEND_AST_EMPTY, $3); }
T_ISSET '(' isset_variables ')' { $$ = $3; }

is_array is an internal function in the core extension standard

/ext/standard/type.c
PHP_FUNCTION(is_array)
{
    php_is_type(INTERNAL_FUNCTION_PARAM_PASSTHRU, IS_ARRAY);
}
MySQL Query : SELECT * FROM `codeshelper`.`v9_news` WHERE status=99 AND catid='6' ORDER BY rand() LIMIT 5
MySQL Error : Disk full (/tmp/#sql-temptable-64f5-1b39e45-2ba18.MAI); waiting for someone to free some space... (errno: 28 "No space left on device")
MySQL Errno : 1021
Message : Disk full (/tmp/#sql-temptable-64f5-1b39e45-2ba18.MAI); waiting for someone to free some space... (errno: 28 "No space left on device")
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