Replacing the line with below line may solve the issue.
$text_slider_content .= '<div class="text_slider_testimonial" style="background-size: cover; background-image: url('. $text_slider_testimonial_img .');">';
Replacing the line with below line may solve the issue.
$text_slider_content .= '<div class="text_slider_testimonial" style="background-size: cover; background-image: url('. $text_slider_testimonial_img .');">';