Detect mysql update/insertion failure due to violated unique constraint

Solution 1:

Now that it's the year 2015, there are very few reasons not to be using PHP's PDO implementation.

The proper, modern, "OO" method for detecting and handling an insertion failure due to a key constraint violation is as follows:

try {
    //PDO query execution goes here.
}
catch (\PDOException $e) {
    if ($e->errorInfo[1] == 1062) {
        //The INSERT query failed due to a key constraint violation.
    }
}

The PDOException object has a lot more to say about the specific nature of the error, too (more detail than one could possibly ever want or need, seemingly).

Solution 2:

http://dev.mysql.com/doc/refman/5.5/en/error-messages-server.html

http://php.net/manual/en/function.mysql-errno.php

I've had to do this in the past, and it's not fun:

if( mysql_errno() == 1062) {
    // Duplicate key
} else {
    // ZOMGFAILURE
}

A note on programming style (Credits to jensgram from this answer)
You should always seek to avoid the use of magic numbers. Instead, you could assign the known error code (1062) to a constant (e.g. MYSQL_CODE_DUPLICATE_KEY). This will make your code easier to maintain as the condition in the if statement is still readable in a few months when the meaning of 1062 has faded from memory :)