PHP class that allows to intercept output to a stream, like STDOUT. Very useful for testing CLI applications using `fwrite('', STDOUT);`.

OutputInterceptor.php

<?php

/**
 * Intercepts bytes written to an output stream and redirects them into a buffer instead
 */
class OutputInterceptor extends php_user_filter
{
    /**
     * Holds the PHP stream filter
     */
    protected static $interceptor;

    /**
     * Holds the output buffer
     */
    protected static $buffer = '';

    /**
     * Reads the captured output and flushes the buffer.
     */
    public static function flush(): string
    {
        $output = static::$buffer;

        static::$buffer = '';

        return $output;
    }

    /**
     * Starts capturing the output on a stream. This has to be called before writing data.
     */
    public static function capture($stream)
    {
        stream_filter_register('intercept', static::class);

        static::$interceptor = stream_filter_append($stream, 'intercept');
    }

    /**
     * Releases the capturing by removing the stream filter. This has to be called after writing data.
     */
    public static function release()
    {
        stream_filter_remove(static::$interceptor);
    }

    /**
     * PHP User Filter handler - this is where the magic happens!
     * Replace PSFS_FEED_ME with PSFS_PASS_ON to achieve a "tee" effect, where the output gets written to
     * both the output stream as well as the buffer.
     */
    public function filter($in, $out, &$consumed, $closing)
    {
        while ($bucket = stream_bucket_make_writeable($in)) {
            static::$buffer .= $bucket->data;
            $consumed += $bucket->datalen;

            stream_bucket_append($out, $bucket);
        }

        return PSFS_FEED_ME;
    }
}