Handle error for duplicate entries - PHP MySQL

Question

I have a PHP form which enters data into my MySQL database. My primary key is one of the user-entered values. When the user enters a value that already exists in the table,

Answer 1

Use mysql_errno() function, it returns the error numbers. The error number for duplicate keys is 1062. for example

$query = mysql_query("INSERT INTO table_name SET ...);
if (mysql_errno() == 1062){
    echo 'Duplicate key';
}