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; }