Simple code to answer, the younger brother really does not understand why the return failed! What should I do?

Simple code to answer, the younger brother really does not understand why the return failed! I want to add data to the database, but it can fail. this is my SQL class. the code below is the code that I call this class. I hope you can help me to answer this question. thank you! & Lt ;? Php & nbsp; classSqlTool!
I want to add data to the database, but it can fail. this is my SQL class. the code below is the code that I call this class. I hope you can help me to answer this question. thank you!
Class SqlTool {
Private $ conn;
Private $ host = "localhost ";
Private $ user = "root ";
Private $ password = "root ";
Private $ db = "test ";

Function SqlTool (){
$ This-> conn = mysql_connect ($ this-> host, $ this-> user, $ this-> password );
If (! $ This-> conn ){
Die ("connect faild! ". Mysql_error ());
}
Mysql_select_db ($ this-> db, $ this-> conn );
Mysql_query ("set names gbk ");
}
// Complete select
// Function execute_dql ($ SQL ){

//}
// Function: update, delete, insert
Function execute_dml ($ SQL ){
$ B = mysql_query ($ SQL );
// Var_dump ($ B );
If (! $ B ){
Return 0; // faild
} Else {
If (mysql_affected_rows ($ this-> conn)> 0 ){
Return 1; // success
} Else {
Return 2; // no influence
}
}
}
}
?>





Require_once "mysqlTool. php ";
// Create mysql
$ SQL = "insert into user1 (name, password, email, age) values ('xiaoming ', md5 (123), 'xiaoming @ sina.com', 20 )";
$ SqlTool = new SqlTool ();
$ Res = $ sqlTool-> execute_dml ($ SQL );
// Var_dump ($ res );
If ($ res = 0 ){
Echo "faild! ";
} Else if ($ res = 1 ){
Echo "success ";
} Else if ($ res = 2 ){
Echo "no influence ";
}
?>







------ Solution --------------------
If the SQL command execution fails.

$ B = mysql_query ($ SQL) or die (mysql_error ());
In this way, an error will be prompted.