mysql_num_rows in the error log

Question

I am encountering an error. My log is as follows:

PHP Warning:  mysql_num_rows(): supplied argument is not 
a valid MySQL result resource in     
/home/domain/public_html/index.php on line 96

My code is as followsL line 96:

mysql_num_rows(mysql_query("SELECT * FROM `table` WHERE 
UNIX_TIMESTAMP(`date`)>$user_last_visit"));

What could cause this error?

Answer 1

You put two functions into each other directly:

mysql_num_rows(mysql_query(...));

Technically this is possible to do (function 1 returns something that becomes the parameter of function 2), however, in case function 1 returns something that function 2 can not deal with, you get errors.

That happens in your case. Instead store the return value into a variable and do error handling your own:

$result = mysql_query(...);
if (!$result) {
    throw new Exception(sprintf('Database query failed: %s', mysql_error()));
}
$num_rows = mysql_num_rows($result);

Proper error handling is crucial for solid programming, so take care about return values. When in doubt double-check for the function in question with the PHP manual which is really a great resource. See mysql_query.