// In your custom plugin or system plugin:
function myPhpShortcode($atts, $content = null) {
    // Execute PHP
    ob_start();
    ?>
    <div>My PHP Output: <?php echo date("Y-m-d"); ?></div>
    <?php
    $output = ob_get_clean();
    return $output;
}
add_shortcode('my_php', 'myPhpShortcode');