PHP:strripos () does not recognize the < input > tag of string type?

question:
I am using the lightning strike net custom form. In the rich text submitted to the backend, I extract the string of the html tag in the rich text string, and then I want to replace the html tag string in the rich text string with a space (that is, delete ~), so I confidently use the strripos (), result to get the return value of false, that is, the string B that clearly exists in string A. But I can"t tell where he is, so I can"t achieve the goal of replacing B.

        /*   */
        foreach ($form as $v) {
            $needle = $v["content"];
//            var_dump($needle);
//            $needle = "<input";
//            echo "=>".$contents." <br> =>".$needle;

            var_dump($contents);
            var_dump($needle);
            $start = stripos($contents, $needle);//TODO 
            if ($start!=false) {
                $length = strlen($needle);
                echo "" . $contents . "--------------$start=" . $start . ",$length=" . $length . "<br>";
                $contents = $this->str_replace_once($needle, "", $contents);// html
//            $contents = substr_replace($contents, "", $start, $length);
//            echo "=" . $contents . "<br>";
            }

        }

above code: replace needle with a blank string from contents.

 var_dump($contents);==>
string(204) "asdfasdf
<input name="leipiNewField" type="text" title="fdasfd" value="" leipiplugins="text" orghide="0" orgalign="left" orgwidth="150" orgtype="text" style="text-align: left; width: 150px;"/>"


 var_dump($needle);==>string(184) "
<input name="leipiNewField1" type="text" title="fdasfd" value="" leipiplugins="text" orghide="0" orgalign="left" orgwidth="150" orgtype="text" style="text-align: left; width: 150px;"/>"

Rich text submitted: