Check to see if the user name already exists in mysql

When writing to the registration system, if you want to check if the user name exists, php can write like this:

 
$username=$_REQUEST['username']; 
$password=$_REQUEST['password']; 
echo "<br/>excute the insertUserInfo"; 
$con=mysqli_connect("localhost","root","","my_db"); 
$sql="SELECT * FROM user WHERE username='$username'"; 

ho "<br/>this username aready exists"; } else { $sql="INSERT INTO user (username,password) VALUES ('$username','$password')"; if(!mysqli_query($con,$sql)) { echo "<br/>Error".mysqli_error($con); } else { echo "<br/>one record added"; } } 

Don't use:

 
mysql_num_rows($result)</pre> 
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