SQL_CALC_FOUND_ROWS vs mysql_num_rows()

What function do you use to return the number of records in a MYSQL table and why?
To understand the difference between SQL_CALC_FOUND_ROWS and mysql_num_rows()

$sql  = mysql_query("
                              SELECT      SQL_CALC_FOUND_ROWS id
                              FROM        table
                              WHERE       id_msg  =  2
                            ");
$pages_resultats	=	mysql_query("SELECT FOUND_ROWS()");
list($nbr_resultats) = mysql_fetch_array($pages_resultats);

Resultats :

echo mysql_num_rows($sql); // Return 4
echo $nbr_resultats; // Return 4

Now add “Limit” to query and retry :

$sql  = mysql_query("
                              SELECT      SQL_CALC_FOUND_ROWS id
                              FROM        table
                              WHERE       id_msg  =  2
                              LIMIT       2
                            ");
$pages_resultats	=	mysql_query("SELECT FOUND_ROWS()");
list($nbr_resultats) = mysql_fetch_array($pages_resultats);

Resultats :

echo mysql_num_rows($sql); // Return 2
echo $nbr_resultats; // Return always 4

Unlike mysql_num_rows() or COUNT(), the function SQL_CALC_FOUND_ROWS is not affected by the LIMIT clause.
SQL_CALC_FOUND_ROWS is ideal for paging navigation.