What are common reasons for the error "Warning: mysql_fetch_array(): supplied argument is not a valid MySQL result" in PHP?

This error typically occurs when the query executed by `mysql_query()` fails for some reason, such as a syntax error or a problem with the database connection. To solve this issue, you should first check if the query is returning a valid result before attempting to fetch data using `mysql_fetch_array()`. You can do this by checking the result of `mysql_query()` against `false` before proceeding with `mysql_fetch_array()`.

// Execute the query
$result = mysql_query($query);

// Check if the query was successful
if($result === false){
    die("Error: " . mysql_error());
}

// Fetch data using mysql_fetch_array()
while($row = mysql_fetch_array($result)){
    // Process the fetched data
}