bohemian rhapsody php empty function instructions

Determine whether a variable is considered to be empty.
But in my memory, for a long time, I always thought that empty should be able to determine whether a thing is empty, until one day I was like this "empty(trim($a ))" when using it, and then I checked the manual and found out that the empty function can only judge whether a variable is empty, but cannot judge whether a string (or other value, including function return value) is empty. Although I knew that this was only a superficial reason, I did not understand the deeper reasons.
I recently became interested in the php kernel, and thought of this problem again, so I wrote the following code based on other people's experience:

Copy the code The code is as follows:

echo "";
$tokens = token_get_all('');
foreach($tokens as $t){
if (is_array($t)){
          printf("%s t %s n", token_name($t[0]), htmlspecialchars($t[1]));                                                                                                                                                                


The result output is as follows: (If you don’t understand this, please take a look at the "List of Parser Tokens" part of the appendix of the php manual)
T_OPEN_TAG T_EMPTY empty

(
T_STRING trim
(
T_VARIABLE $a
)
)
T_CLOSE_TAG ?>
The token corresponding to the variable should be T_VARIABLE, and trim($a) corresponds to T_STRING. So if you run "empty(trim($a));" directly, it will cause a running error:
Fatal error: Can't use function return value in write context
What happens if I want to empty a string directly later? The result is not a running error, but a parsing error:
Parse error: parse error, expecting `T_STRING' or `T_VARIABLE' or `'$'' in...
(This error message seems annoying...)
The token corresponding to the string is T_CONSTANT_ENCAPSED_STRING (string syntax). Although the parameters are different for the token, they are not T_VARIABLE. An error should have occurred during the compilation stage. Why. . .
So I followed the example of those experts and checked the source code of php. . . But I haven’t read the relevant code yet, haha, I’ll share it next time~
One last time, empty is a language structure, not a function!
(Like print, echo, include, require, die, etc., they are all language structures. Some articles on the Internet say that if print has a return value, it is wrong to call it a function)
The above introduces the instructions for using the bohemian rhapsody php empty function, including the content of bohemian rhapsody. I hope it will be helpful to friends who are interested in PHP tutorials.