Below is my code can you please help me to fix it.

function gw_bbcode_html($t)
    $regexfind[] = '/\<(.+)\>/esiU';
    $t = str_replace('&', '&', $t);
    $t = str_replace('"', '"', $t);
    $t = str_replace('&039;', '\'', $t);
    $regexreplace[] = "gw_bbcode_html_tag(gw_bbcode_htmlspecialchars('\\1'))";
    $t = preg_replace_callback($regexfind, $regexreplace, $t);
#   $t = str_replace('&#', '&#', $t);
    $t = str_replace('![cdata', '![CDATA', $t);
    $t = str_replace('!doctype', '!DOCTYPE', $t);
    $t = str_replace("\t", '       ', $t);
    $t = str_replace("\n", '<br />', $t);
    $t = str_replace('  ', '&#160;&#160;', $t);
    return $t;

Hi I am using PHP Version 5.5.15 and the error is "preg_replace_callback(): Requires argument 2, 'Array', to be a valid"

Maybe you don't want to use a callback? I'm guessing here since the error message still looks truncated.
Read https://stackoverflow.com/questions/44602874/warning-preg-replace-callback-requires-argument-2-to-be-a-valid-callback where it's covered by a reply with "You cannot use a plain text string with variables in it, it needs to be a callable (function), please check the documentation for more information."

Since you know what your code was supposed to do, change to another function or change the variables to callable functions.