Instructions for using php empty function_PHP tutorial

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( An error occurred when using $a))", and then I checked the manual and found out that the empty function can only determine whether a variable is empty, but cannot determine 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 code Code As follows:

echo "

"; <br>$tokens = token_get_all('<?php empty(trim($a));?&gt ;'); <BR>foreach($tokens as $t){ <br> if (is_array($t)){ <br> printf("%s t %s n", token_name($t[0]), htmlspecialchars ($t[1])); <br> }else{ <br> } printf("t%sn", $t); <br> } <br>} <br>

The result output is as follows : (If you don’t understand this, please take a look at the “List of Parser Tokens” section in 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 directly empty a string later? Woolen cloth? 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 feels confusing . . . )
The token corresponding to the view 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 looked at 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)

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