Keeps a clean separation between legitimate content and error messages

php_emit.php

#!/usr/bin/env php
<?php

$_emit_shutdown_function_registered = false;

/**
 * Emits (using echo()) the return value of a closure function.
 * Keeps a clean separation between legitimate content and error messages.
 *
 * If any type of fatal error or Exception was encountered during evaluation, 
 * the HTTP status code will be set to 500 and the error message will be emitted in plain text.
 *
 * Ouput-modifying side effects such as echo() calls or PHP warnings will be swallowed
 * and dumped to the error log instead.
 * 
 * Example:
 *
 *   emit(function() {
 *     return "foo bar\n";
 *   });
 *
 * @param $fn a Closure returning the value to emit.
 */
function emit($fn) {
    global $_emit_shutdown_function_registered;
    $error_mask = E_ERROR | E_CORE_ERROR | E_USER_ERROR;
    $saved_error_flags = error_reporting();

    // Suppress fatal errors in output, since it's already going to be emitted by shutdown function
    error_reporting($saved_error_flags & ~$error_mask);

    ob_start();

    // Emit PHP fatal error
    if (!$_emit_shutdown_function_registered) {
        register_shutdown_function(function() use ($error_mask) {
            $error = error_get_last();
            $type = $error['type'];

            // Ignore warnings
            if (($error_mask & $type) === 0)
                return;

            if ($error !== NULL) {
                $error_map = array(
                    E_ERROR                 => 'E_ERROR',
                    E_WARNING               => 'E_WARNING',
                    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'
                );

                $error_type = $error_map[$type];
                if (!$error_type)
                    $error_type = $type;

                $message = 'Error ' . $error_type . ' (in ' . $error['file'] . 
                           ', line ' . $error['line'] . '): ' . $error['message'];
            }
            else {
                $message = 'Error: Unknown error occurred';
            }

            ob_clean();
            debug_print_backtrace();
            $message .= "\n" . ob_get_clean();

            http_response_code(500);
            header('Content-Type: text/plain');
            echo $message;
            //die();
        });
    }

    try {
        $result = $fn();

        // Swallow warnings in output, logging them instead
        $output = ob_get_clean();
        if ($output) {
            $output = strip_tags(preg_replace('/\<br(\s*)?\/?\>/i', "\n", $output));
            error_log('emit() got unexpected PHP output: ' . $output);
        }
        echo $result;
    }
    // Emit error on raised exceptions
    catch (Exception $ex) {
        ob_clean();
        debug_print_backtrace();
        $message = get_class($ex) . ': ' . $ex->getMessage() . "\n" . ob_get_clean();

        http_response_code(500);
        header('Content-Type: text/plain');
        echo $message;
    }

    error_reporting($saved_error_flags);
}

?>