定義済み定数

-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) {        echo array_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;  $i++ ) {
    print FriendlyErrorType($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 "";
}
?>

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)    ));?>

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.

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']);}

Since PHP 8.4.0:E_STRICT (2048) has been removed.E_ALL is now equal to 30719

<?php
function getErrorTypeByValue($type) {
    $constants  = get_defined_constants(true);

    foreach ( $constants['Core'] as $key => $value ) { // Each Core constant
        if ( preg_match('/^E_/', $key  ) ) {    // Check error constants
            if ( $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]