To pass more than one css rule, but also a class, an id, or any html attribute to highlighted element, we can escape a quote to close the inline styling:
    ini_set('highlight.string', '#F8F8F8 ; font-size:1.4em\" class=\'string\' ');The behaviour of these functions is affected by settings in php.ini.
| Name | Default | Changeable | Changelog | 
|---|---|---|---|
| ignore_user_abort | "0" | INI_ALL | |
| highlight.string | "#DD0000" | INI_ALL | |
| highlight.comment | "#FF8000" | INI_ALL | |
| highlight.keyword | "#007700" | INI_ALL | |
| highlight.default | "#0000BB" | INI_ALL | |
| highlight.html | "#000000" | INI_ALL | |
| browscap | NULL | INI_SYSTEM | 
Here's a short explanation of the configuration directives.
ignore_user_abort
     bool
    
      false by default. If changed to true scripts will not be terminated
      after a client has aborted their connection.
     
See also ignore_user_abort().
highlight.bg
      string
     highlight.comment
      string
     highlight.default
      string
     highlight.html
      string
     highlight.keyword
      string
     highlight.string
      string
     Colors for Syntax Highlighting mode. Anything that's acceptable in <font color="??????"> would work.
browscap
     string
    Name (e.g.: browscap.ini) and location of browser capabilities file. See also get_browser().
To pass more than one css rule, but also a class, an id, or any html attribute to highlighted element, we can escape a quote to close the inline styling:
    ini_set('highlight.string', '#F8F8F8 ; font-size:1.4em\" class=\'string\' ');*** Additions to the previous message ***
To change the style directly from a CSS file, it is recommended to only go through classes instead of hard-coding it.
<?php
    // Create an array with default php functions
    $functions = array("default", "html", "keyword", "string", "comment");
    // Replace color code (ex: #FF8000) with class name (ex:"highlight-comment")
    foreach ($functions as $value) {
        ini_set("highlight.$value", "highlight-$value;");
    }
    $content = highlight_file($filename, true);
    // or
    $content = highlight_string($string, true);
    // Convert style="color: highlight-function" into class="highlight-function" into $content
    // It allows you to modify only the default functions
    foreach ($functions as $value) {
        $content = preg_replace("/style=\"color: highlight-$value;\"/", "class=\"highlight-$value\"", $content);
    }
?>
And in the CSS file (for example) :
.highlight-html { color: #000000; }
.highlight-default { color: #0000bb; }
.highlight-keyword { color: #007700; font-weight: bold; }
.highlight-string { color: #dd0000; }
.highlight-comment { color: #ff8000; }To change the style directly from a CSS file, it is recommended to only go through classes instead of hard-coding it.
<?php
    $functions = array("default", "html", "keyword", "string", "comment");
    foreach ($functions as $value) {
        ini_set("highlight.$value", "highlight-$value;");
    }
?>
And in the CSS file (for example) :
.highlight-html { color: #000000; }
.highlight-default { color: #0000bb; }
.highlight-keyword { color: #007700; font-weight: bold; }
.highlight-string { color: #dd0000; }
.highlight-comment { color: #ff8000; }