Why exactly does this code do? And what kind of operator is &?

Question

I was searching the manual for error_reporting. There I found an example which lists all the error reporting levels in the browser window. I went through the code and some coding style seems unfamiliar to me and I couldn't understand what exactly it does.

In the manual, it says error_reporting () returns:

Returns the old error_reporting level or the current level if no level parameter is given

I know error_reporting(level) determines what type of error to show or not to show. But what it returns when it is assigned to a variable ? like the following:

$errLvl = error_reporting();

I have printed $errLvl and it returned 22527. I don't understand what it means? Can any one explain it to me in plain English?

There is a function called FriendlyErrorType($type) which will accept an error type and print it in the browser. But it is given a parameter ($errLvs & pow(2,i)). What does & operator supposed to do? I mean what is the function of & operator in the argument?

print FriendlyErrorType($errLvl & pow(2, $i))

fullCode:

$errLvl = error_reporting();
echo $errLvl.'</br>';
for ($i = 0; $i < 15;  $i++ ) {
    print FriendlyErrorType($errLvl & pow(2, $i)) . "<br>\\n";
}

function FriendlyErrorType($type)
{
    switch($type)
    {
        case E_ERROR: // 1 //
            return 'E_ERROR';
        case E_WARNING: // 2 //
            return 'E_WARNING';
        case E_PARSE: // 4 //
            return 'E_PARSE';
        case E_NOTICE: // 8 //
            return 'E_NOTICE';
        case E_CORE_ERROR: // 16 //
            return 'E_CORE_ERROR';
        case E_CORE_WARNING: // 32 //
            return 'E_CORE_WARNING';
        case E_COMPILE_ERROR: // 64 //
            return 'E_COMPILE_ERROR';
        case E_COMPILE_WARNING: // 128 //
            return 'E_COMPILE_WARNING';
        case E_USER_ERROR: // 256 //
            return 'E_USER_ERROR';
        case E_USER_WARNING: // 512 //
            return 'E_USER_WARNING';
        case E_USER_NOTICE: // 1024 //
            return 'E_USER_NOTICE';
        case E_STRICT: // 2048 //
            return 'E_STRICT';
        case E_RECOVERABLE_ERROR: // 4096 //
            return 'E_RECOVERABLE_ERROR';
        case E_DEPRECATED: // 8192 //
            return 'E_DEPRECATED';
        case E_USER_DEPRECATED: // 16384 //
            return 'E_USER_DEPRECATED';
    }
    return "";

i know that .But here we have to send an argument.What an bitwise operator needs to be there? — AL-zami, Apr 28 '15 at 17:57
perhaps edit your question to show (clarify) that you understand what the `&` operator *does* and what it *is*, but not actually why it is required in that part of that code? — Martin, Apr 28 '15 at 17:58
@Lashane ahh ok thanks for the correction. (I removed the comment) — Martin, Apr 28 '15 at 17:59
@AL-zami read http://en.wikipedia.org/wiki/Mask_%28computing%29 it will give you some understanding what is mask — Iłya Bursov, Apr 28 '15 at 17:59
also http://en.wikipedia.org/wiki/Bit_field could give some details — Iłya Bursov, Apr 28 '15 at 18:00

score 5 · Accepted Answer · edited May 23 '17 at 11:50

So your code is just checking which error types you have on and returns the constant name of that error type.

But first to clarify what & operator this is. It is a bitwise AND operator (Also see this as reference: Reference - What does this symbol mean in PHP?).

So basically it does this:

  A    |    B    &   results 
-----------------------------
  0    |    0   ->     0
  0    |    1   ->     0
  1    |    1   ->     1
  1    |    0   ->     0

So a plain english example woulde be:

If the sun is shining AND it I don't have to work at this day THEN I go outside (otherwise NOT).

So now your code just get's your current error level:

$errLvl = error_reporting();

For you it is:

22527  //Which in binary is: 0101'0111 1111'1111

After this it goes through the loop 15 times and basically always checks if the next digit is 1 or 0, e.g:

1. Iteration:

0101'0111 1111'1111            // your error level
                  1            // pow(2, $i) -> 2^0 -> 1
------------------- &
                  1  =      1  //argument for the function call

2. Iteration:

0101'0111 1111'1111            // your error level
                 10            // pow(2, $i) -> 2^1 -> 2
------------------- &
                 10  =      2  //argument for the function call

...

15. Iteration:

0101'0111 1111'1111            // your error level
0100'0000 0000'0000            // pow(2, $i) -> 2^14 -> 16384
------------------- &
0100'0000'0000'0000  = 16'384  //argument for the function call

And then in the function it simply is a switch statement to get the right term to return if it the corresponding digit is set.

You can also see all predefined error constants here: http://php.net/manual/en/errorfunc.constants.php

Here visualized:

0000'0000 0000'0000
|||| |||| |||| |||| ----- E_ERROR              =      1
|||| |||| |||| ||| ------ E_WARNING            =      2
|||| |||| |||| || ------- E_PARSE              =      4
|||| |||| |||| | -------- E_NOTICE             =      8
|||| |||| ||||
|||| |||| |||| ---------- E_CORE_ERROR         =     16
|||| |||| ||| ----------- E_CORE_WARNING       =     32
|||| |||| || ------------ E_COMPILE_ERROR      =     64
|||| |||| | ------------- E_COMPILE_WARNING    =    128
|||| ||||
|||| |||| --------------- E_USER_ERROR         =    256
|||| ||| ---------------- E_USER_WARNING       =    512
|||| || ----------------- E_USER_NOTICE        =  1'024
|||| | ------------------ E_STRICT             =  2'048
||||
|||| -------------------- E_RECOVERABLE_ERROR  =  4'096
||| --------------------- E_DEPRECATED         =  8'192
|| ---------------------- E_USER_DEPRECATED    = 16'384
| ----------------------- E_ALL                = 32'767

Now if you want to ask why it doesn't check for the last digit (E_ALL), because basically E_ALL == all error types. So if you get all error types back you have error reporting: E_ALL

So for the end your error level visualized:

0101'0111 1111'1111
 | |  ||| |||| |||| ----- E_ERROR         
 | |  ||| |||| ||| ------ E_WARNING          
 | |  ||| |||| || ------- E_PARSE   
 | |  ||| |||| | -------- E_NOTICE   
 | |  ||| ||||
 | |  ||| |||| ---------- E_CORE_ERROR      
 | |  ||| ||| ----------- E_CORE_WARNING   
 | |  ||| || ------------ E_COMPILE_ERROR    
 | |  ||| | ------------- E_COMPILE_WARNING 
 | |  |||
 | |  ||| --------------- E_USER_ERROR   
 | |  || ---------------- E_USER_WARNING     
 | |  | ----------------- E_USER_NOTICE    
 | |
 | | -------------------- E_RECOVERABLE_ERROR  
 |  
 | ---------------------- E_USER_DEPRECATED   



output of your code:

E_ERROR 
E_WARNING
E_PARSE  
E_NOTICE
E_CORE_ERROR 
E_CORE_WARNING  
E_COMPILE_ERROR 
E_COMPILE_WARNING 
E_USER_ERROR  
E_USER_WARNING
E_USER_NOTICE
E_RECOVERABLE_ERROR  
E_USER_DEPRECATED

Why exactly does this code do? And what kind of operator is &?

1 Answers1