strip_tags converts HTML to plain text. If you're typing "Mileage <= 15000", that's not HTML, it's text, so strip_tags is the wrong function. Use htmlspecialchars.

This text can be in comments. Some of this comments can be from user. There can be tags that I must strip.  But there can be "Mileage <= 15000" that I shouldn't streep. What should I do?

>There can be tags that I must strip.

In other words, you're trying to make it a nuisance for anyone on the off chance they want to talk about HTML.

strip_tags is the wrong function. Use htmlspecialchars.

In security reasons (to prevent XSS) I should strip all tags in user comments. It's not a developer blog and there wont be conversation about HTML. 
And your comment "they want to talk about HTML." is not related to the subject.
The subject is "The function strip_tags should strip tags and only tags".

There's a very easy way to prevent XSS and allow "Mileage <= 15000". It's called htmlspecialchars.

Ok, htmlspecilachars will prevent XSS. But I want to STRIP TAGS. 

I want to see 
"Please use google.com"
when user write
"<b>Please</b> use <a href="https://www.google.com">google.com</a>"
AND NOT an encoded variant of "<b>Please</b> use <a href="https://www.google.com">google.com</a>" as htmlspecilachars do.

Why would someone write that?

Oh wait, you just did. Can you imagine how annoying it would be for you to type your comment if this bug page used strip_tags?

The subject is "The function strip_tags should strip tags and only tags".
Not "How I use strip_tags". Or "how somebody filter comments". 
So if you can't write something related to the subject please do not spam.

The subject is that you're misusing the function. strip_tags removes tags from HTML. This is not valid HTML:

    Mileage <= 15000

Now read the warning in the documentation: 

    Because strip_tags() does not actually validate the HTML, partial or
    broken tags can result in the removal of more text/data than expected.

I.e., garbage in = garbage out.

Browsers are quite error tolerant in parsing HTML. strip_tags tries to be secure with the risk of breaking input. If you want to take data into HTML and want the browser to render it properly use htmlentities etc.

I want to filter string writen by user
"<b>Best car ever!<b/> Mileage <= 15000"
so it looks like
"Best car ever! Mileage <= 15000";
I want to strip only tags. PHP can't do it using native functions?

This is not a support forum. There is no bug here.