Undefined variable in IF statement (but seems correctly declared) - php

Question

Something makes me crazy, I get an Undefined variable if I declare a variable in an IF statement. If I declare this variable above the IF statement, the PHP Notice disapear.
My code :
if($tab_sel==FILENAME_DEFAULT){ // FILENAME_DEFAULT = index.php
if(...){
some functions
}
$nb_pages=ceil($count/$limit) : 0;
}
echo $nb_pages; //4000 is displayed on the page (OK),
but in the Nginx log, I get :
FastCGI sent in stderr: "PHP message: PHP Notice : Undefined variable nb_pages in /.../index.php
but when I load the page, I can see that nb_page is displayed correctly (example: value is 4000), but in the log, undefined variable..
In my example, if I declare nb_pages above "if($tab_sel==FILENAME_DEFAULT){", the undefined disapear.
Any idea?

Answer 1

That's because you're creating the variable inside a if that can fail to pass. In this case, the variable would be undefined and PHP will throw an error since it couldn't find the variable that you are asking for afterwards.
Declare it before the if and you will be just fine.
$nb_pages;
if($tab_sel==FILENAME_DEFAULT){
if(...){

Answer 2

I think it might me an environment issue. Try to put both FILENAME_DEFAULT and $nb_pages into a log file:
file_put_contents(
'test.log',
date('[Y-m-d H:i:s] ')
. FILENAME_DEFAULT . "\t" . #$nb_pages . PHP_EOL,
FILE_APPEND
);
The # will prevent the warning message.

Answer 3

I found why it didn't work :
in case of a 404 error page (called FILENAME_404)
I also use the index.php file in which I call the variables undeclared.
that's why the Nginx error said about index.php, not because it doesn't go into if($tab_sel==FILENAME_DEFAULT){ but because in some other cases I use also index.php with tab_sel=FILENAME_404
thank you everyone!