Here's a definitive solution, which supports negative character classes and the four documented flags.
<?php
    
    if (!function_exists('fnmatch')) {
        define('FNM_PATHNAME', 1);
        define('FNM_NOESCAPE', 2);
        define('FNM_PERIOD', 4);
        define('FNM_CASEFOLD', 16);
        
        function fnmatch($pattern, $string, $flags = 0) {
            return pcre_fnmatch($pattern, $string, $flags);
        }
    }
    
    function pcre_fnmatch($pattern, $string, $flags = 0) {
        $modifiers = null;
        $transforms = array(
            '\*'    => '.*',
            '\?'    => '.',
            '\[\!'    => '[^',
            '\['    => '[',
            '\]'    => ']',
            '\.'    => '\.',
            '\\'    => '\\\\'
        );
        
        if ($flags & FNM_PATHNAME) {
            $transforms['\*'] = '[^/]*';
        }
        
        if ($flags & FNM_NOESCAPE) {
            unset($transforms['\\']);
        }
        
        if ($flags & FNM_CASEFOLD) {
            $modifiers .= 'i';
        }
        
        if ($flags & FNM_PERIOD) {
            if (strpos($string, '.') === 0 && strpos($pattern, '.') !== 0) return false;
        }
        
        $pattern = '#^'
            . strtr(preg_quote($pattern, '#'), $transforms)
            . '$#'
            . $modifiers;
        
        return (boolean)preg_match($pattern, $string);
    }
    
?>
This probably needs further testing, but it seems to function identically to the native fnmatch implementation.