mb_substr, mb_strlng, mb_sterlen, mb_strpos errors

$append=mb_substr($messagetopost, 0,mb_strpos($messagetopost, $val));

				if(mb_strlen($append)<($len-mb_strlen($substring)))
				{
					$substring.=$append;
				}
				else if($issubstr==0)
				{
					$avl=$len-mb_strlen($substring)-4;
					if($avl>0)
						$substring.=mb_substr($append, 0,$avl)."...";

					$issubstr=1;

				}

is there any solutions for avoid these codes? I’ve FB auto post working but twitter is giving these errors.
Should you put some fixed codes for untouched characters with internal encodin etc ?

https://www.ads-software.com/plugins/social-media-auto-publish/