Query empty if included - php

Question

I have been performing a query inside my page -- say, page.php -- where I run a simple query.
Pseudo-code:
$request_unavailble = mysqli_query($mysqli, "SELECT * FROM my_table WHERE availble='0'");
When this is performed from within page.php, I get all results where availble is set to 0. However, if I run this from within a seperate included file, the data returns empty. In fact, mysqli_num_rows returns 0 when included.
What's going wrong, here?
Edit
The following function was added as an include (both as a function and alone)
function compte_messagerie()
{
$requetes_messagerie = mysqli_query($mysqli, "SELECT * FROM ".DB_PREFIX."messagerie WHERE lu='0'");
if(mysqli_num_rows($requetes_messagerie) == 0)
{
echo '<a id="messagerie" href="messagerie">'.AUCUN_NOUVEAU."</a>";
}
else if(mysqli_num_rows($requetes_messagerie) == 1)
{
echo '<a id="messagerie" href="messagerie">';
echo '<span>'.mysqli_num_rows($requetes_messagerie)."</span> ";
echo MESSAGES_SINGULIER."</a>";
}
else
{
echo '<a id="messagerie" href="messagerie">';
echo '<span>'.mysqli_num_rows($requetes_messagerie)."</span> ";
echo MESSAGES_PLURIEL."</a>";
}
}

Answer 1

When porting your query into a function, the MySQLi connection object in $mysqli went out of scope, and was therefore invalid inside the function. With display_errors enabled, I would expect you to see errors like:
Notice: undefined variable $mysqli
Warning: mysqli_query() expects parameter 1 to be resource, null given
The cleanest solution is to pass $mysqli into your function as a parameter, making it available to the function's scope
// Expect the MySQLi resource as a parameter...
function compte_messagerie($mysqli)
{
$requetes_messagerie = mysqli_query($mysqli, "SELECT * FROM ".DB_PREFIX."messagerie WHERE lu='0'");
if(mysqli_num_rows($requetes_messagerie) == 0)
{
echo '<a id="messagerie" href="messagerie">'.AUCUN_NOUVEAU."</a>";
}
// etc.....
}

Answer 2

Try this. Please check if the available table is require string value or integer.
$request_unavailble = mysqli_query("SELECT * FROM my_table WHERE availble= 0 ");
while($rows = mysqli_fetch_assoc($request_unavailble)){ // <- this will check if there some data fetch
// You put some code here
}