! About the "+" number in phpurlencode

Help! The & quot; + & quot; in phpurlencode is URL encoded and decoded with urldecode '', I don't know whether it is blank or empty. it seems like a space. In short, it's not the original '+' number. I should ask for help! About the "+" number in php urlencode
After the "+" in php is encoded by urlencode,


The URL is decoded with urldecode and changed to '', which is null or empty. it seems like a space,

In short, it is not the original '+' number. how can I solve this problem to get the original "+" number?

------ Solution --------------------
Discussion

This also occurs in my tests:
PHP code

# Sending page
$ Param = "1 + 1 ";
Echo urlencode ($ param )."
";
# 1% 2B1
Header ("Location:./test17.php /? Can = ". urlencode ($ param ));
# Http: // path/test/test_csdn/test17.php /? Can = 1% 2B1

# Receiving Page
I ......

------ Solution --------------------
Discussion

So what is the use of this urldecode function?