PHP : Notice: Undefined variable

I have this error

Notice: Undefined variable: s in xxx

how to fix this?
Answer:
This come out because we use/type like this

$s.=bla bla;

 meanwhile.. you not declare the variable. PHP allow you to type directly like that, but for security issue..

You should declare first like this

$s="";
$s.=$a[one]."<br>";
$s.=$a['one']."<br>";

die($s);

 The error show because you using error_reporting(); If you can't handle all the error, try lower the reporting to 1 or 0!!
0: no report