Detect unclosed html tag in string

Hey guys can anyone help me to get ride of unclosed tags in string? my input have +20000 length of strings, the service i use (telegram) limited to 4096 bytes, So i used str_split($input, 4000), The problem is my strings contains html tag i mean:

<b>My headline </b>

dolor sit amet, consec
quis nostrud exer
xcepteur sint

<b>My Second headline</b>

w dfficia deserunt
in voluptate velit
xcepteur sint

<b>My third headline</b>

another 500 headlines with b tag

If my input have unclosed tags the telegram api complains about it.

So when i splited them, some of the become > ... </b> or b>.... </b> , ... </b>, <b>...<

How can i detect and delete unclosed html tag after str_split?