-1 is also semantically meaningless as a bit field, and only works in 2s-complement numeric representations.  On a 1s-complement system -1 would not set E_ERROR.  On a sign-magnitude system -1 would set nothing at all! (see e.g.http://en.wikipedia.org/wiki/Ones%27_complement)If you want to set all bits, ~0 is the correct way to do it.But setting undefined bits could result in undefined behaviour and that means *absolutely anything* could happen :-)

An other way to get all PHP errors  that are set to be reported. This code will even work, when additional error types are added in future.<?php$pot=0;foreach (array_reverse(str_split(decbin(error_reporting()))) as$bit) {    if ($bit==1) {        echoarray_search(pow(2,$pot),get_defined_constants(true)['Core'])."<br>\n";    }$pot++;}?>

[Editor's note: fixed E_COMPILE_* cases that incorrectly returned E_CORE_* strings. Thanks josiebgoode.]The following code expands on Vlad's code to show all the flags that are set.  if not set, a blank line shows.<?php$errLvl=error_reporting();for ($i=0;$i<15;$i++ ) {    printFriendlyErrorType($errLvl&pow(2,$i)) ."<br>\\n"; }functionFriendlyErrorType($type){    switch($type)    {        caseE_ERROR:// 1 //return'E_ERROR';        caseE_WARNING:// 2 //return'E_WARNING';        caseE_PARSE:// 4 //return'E_PARSE';        caseE_NOTICE:// 8 //return'E_NOTICE';        caseE_CORE_ERROR:// 16 //return'E_CORE_ERROR';        caseE_CORE_WARNING:// 32 //return'E_CORE_WARNING';        caseE_COMPILE_ERROR:// 64 //return'E_COMPILE_ERROR';        caseE_COMPILE_WARNING:// 128 //return'E_COMPILE_WARNING';        caseE_USER_ERROR:// 256 //return'E_USER_ERROR';        caseE_USER_WARNING:// 512 //return'E_USER_WARNING';        caseE_USER_NOTICE:// 1024 //return'E_USER_NOTICE';        caseE_STRICT:// 2048 //return'E_STRICT';        caseE_RECOVERABLE_ERROR:// 4096 //return'E_RECOVERABLE_ERROR';        caseE_DEPRECATED:// 8192 //return'E_DEPRECATED';        caseE_USER_DEPRECATED:// 16384 //return'E_USER_DEPRECATED';    }    return"";}?>

A simple and neat way to get the error level from the error code. You can even customize the error level names further.<?php$exceptions= [E_ERROR=>"E_ERROR",E_WARNING=>"E_WARNING",E_PARSE=>"E_PARSE",E_NOTICE=>"E_NOTICE",E_CORE_ERROR=>"E_CORE_ERROR",E_CORE_WARNING=>"E_CORE_WARNING",E_COMPILE_ERROR=>"E_COMPILE_ERROR",E_COMPILE_WARNING=>"E_COMPILE_WARNING",E_USER_ERROR=>"E_USER_ERROR",E_USER_WARNING=>"E_USER_WARNING",E_USER_NOTICE=>"E_USER_NOTICE",E_STRICT=>"E_STRICT",E_RECOVERABLE_ERROR=>"E_RECOVERABLE_ERROR",E_DEPRECATED=>"E_DEPRECATED",E_USER_DEPRECATED=>"E_USER_DEPRECATED",E_ALL=>"E_ALL"];echo$exceptions["1"];$code=256;echo$exceptions[$code];?>Output:  E_ERRORE_USER_ERRORThis will need updating when PHP updates the error level names. Otherwise, it works just fine.

A neat way to have a place in code to control error reporting configuration :)<?php$errorsActive= [E_ERROR=>FALSE,E_WARNING=>TRUE,E_PARSE=>TRUE,E_NOTICE=>TRUE,E_CORE_ERROR=>FALSE,E_CORE_WARNING=>FALSE,E_COMPILE_ERROR=>FALSE,E_COMPILE_WARNING=>FALSE,E_USER_ERROR=>TRUE,E_USER_WARNING=>TRUE,E_USER_NOTICE=>TRUE,E_STRICT=>FALSE,E_RECOVERABLE_ERROR=>TRUE,E_DEPRECATED=>FALSE,E_USER_DEPRECATED=>TRUE,E_ALL=>FALSE,];error_reporting(array_sum(array_keys($errorsActive,$search=true)    ));?>

As for me, the best way to get error name by int value is that. And it's works fine for me ;)<?phparray_flip(array_slice(get_defined_constants(true)['Core'],1,15,true))[$type];//the same in readable formarray_flip(array_slice(get_defined_constants(true)['Core'],1,15,true))[$type]?>

super simple error code to human readable conversion:function prettycode($code){    return $code == 0 ? "FATAL" : array_search($code, get_defined_constants(true)['Core']);}

<?phpfunctiongetErrorTypeByValue($type) {$constants=get_defined_constants(true);    foreach ($constants['Core'] as$key=>$value) {// Each Core constantif (preg_match('/^E_/',$key) ) {// Check error constantsif ($type==$value)                 return("$key=$value");        }    }}// getErrorTypeByValue()echo"[".getErrorTypeByValue(1) ."]".PHP_EOL;echo"[".getErrorTypeByValue(0) ."]".PHP_EOL;echo"[".getErrorTypeByValue(8) ."]".PHP_EOL;?>Will give    [E_ERROR=1]    []    [E_NOTICE=8]