Сергей Харчевников

Можно и без буфера обойтись, просто записывая всё в переменную. Ваш код:

function getShortcodeOutput() {
    ob_start();
    echo "some text1\n";
    echo "some text2\n";
    echo "some text3\n";
    echo "some text4\n";
    $result = ob_get_contents();
    ob_end_clean();
    return $result;
}

Поменяв на:

function getShortcodeOutput() {
    $result = "";
    $result .= "some text1\n";
    $result .= "some text2\n";
    $result .= "some text3\n";
    $result .= "some text4\n";
    return $result;
}

И получиться:

function shortcodeSliderInit($atts) {

	$result = '';
	extract(shortcode_atts(array(
		'name' => '',
		'slideshow' => 0,
		'animation' => 'fade',
		'controlnav' => 0,
		'randomize' => 0
	), $atts));

	if (!$name) { return FALSE; }
	$slider = ale_sliders_get_slider($name);
	if (!$slider) { return FALSE; }
	$slider_id = sanitize_title('ale-slider-' . $slider['slug'] . '-' . wp_generate_password(5, false));

	$result .= '<div id="' . $slider_id . '" class="ale-slider-slides">';
	$result .= '<div class="owl-carousel owl-theme">';
	foreach ($slider['slides'] as $slide) {
		if ($slide['image']) {

			if ($slide['url']) {
				$result .= '<a href="' . $slide['url'] . '" class="swipebox">';
			}

			$result .= '<img src="' . $slide['image'] . '" alt="' . $slide['title'] . '" />';

			if ($slide['url']) {
				$result .= '</a>';
			}
			
		}
		
	}
	$result .= '</div>';
	$result .= '</div>';

	return $result;
}