src/Composer/Util/ErrorHandler.php
public static function handle($level, $message, $file, $line)
    {
        // error code is not included in error_reporting
        if (!(error_reporting() & $level)) {
            return;
        }

        if (filter_var(ini_get('xdebug.scream'), FILTER_VALIDATE_BOOLEAN)) {
            $message .= "\n\nWarning: You have xdebug.scream enabled, the warning above may be".
            "\na legitimately suppressed error that you were not supposed to see.";
        }

        if ($level !== E_DEPRECATED && $level !== E_USER_DEPRECATED) {
            throw new \ErrorException($message, 0, $level, $file, $line);
        }

        if (self::$io) {
            self::$io->writeError('<warning>Deprecation Notice: '.$message.' in '.$file.':'.$line.'</warning>');
            if (self::$io->isVerbose()) {
                self::$io->writeError('<warning>Stack trace:</warning>');
                self::$io->writeError(array_filter(array_map(function ($a) {
                    if (isset($a['line'], $a['file'])) {
                        return '<warning> '.$a['file'].':'.$a['line'].'</warning>';
                    }

                    return null;
                }, array_slice(debug_backtrace(), 2))));
            }
        }
    }
public static function handle($level, $message, $file, $line)
    {
        // error code is not included in error_reporting
        if (!(error_reporting() & $level)) {
            return;
        }

        if (filter_var(ini_get('xdebug.scream'), FILTER_VALIDATE_BOOLEAN)) {
            $message .= "\n\nWarning: You have xdebug.scream enabled, the warning above may be".
            "\na legitimately suppressed error that you were not supposed to see.";
        }

        if ($level !== E_DEPRECATED && $level !== E_USER_DEPRECATED) {
            throw new \ErrorException($message, 0, $level, $file, $line);
        }

        if (self::$io) {
            self::$io->writeError('<warning>Deprecation Notice: '.$message.' in '.$file.':'.$line.'</warning>');
            if (self::$io->isVerbose()) {
                self::$io->writeError('<warning>Stack trace:</warning>');
                self::$io->writeError(array_filter(array_map(function ($a) {
                    if (isset($a['line'], $a['file'])) {
                        return '<warning> '.$a['file'].':'.$a['line'].'</warning>';
                    }

                    return null;
                }, array_slice(debug_backtrace(), 2))));
            }
        }
    }
Error handler

@param int    $level   Level of the error raised
@param string $message Error message
@param string $file    Filename that the error was raised in
@param int    $line    Line number the error was raised at

@static
@throws \ErrorException