In order to start building the "TemplateProcessor" class, the first thing I'll do is show the definition of its constructor. The signature for this class method is shown below. 

public function __construct($tags=array()){
    if(count($tags)<1){
            throw new Exception('Invalid number of tags');
    }
    if($this->isCacheValid()){
        // read data from cache file
        $this->output=$this->readCache();
    }
    else{
        $this->tags=$tags;
        // read template file
        $this->output=file_get_contents($this->templateFile);
        // process template file
        $this->processTemplate($this->tags);
        // clean up empty tags
        $this->output=preg_replace("/{w}|}/",'',$this->output);
        // write compressed data to cache file
        $this->writeCache();
    }
    // send gzip encoding http header
    $this->sendEncodingHeader();
}

As shown above, the constructor performs some crucial tasks, after checking the validity of the $tags array passed as a parameter. Essentially, this method first determines whether the corresponding cache file is valid, and in accordance with this, reads the parsed web page from the cache.

On the other hand, if the cache has expired, then the method reads the template file in question, then processes it by the private "processTemplate()" method, and finally saves the parsed page to the cache file. Naturally, this operation is performed by the private "writeCache()" method.

The ending line of the constructor calls up the "sendEncodingHeader()" method, in order to tell the browser that data will be sent encoded. Short and understandable, right?

After explaining the logic implemented by the constructor, I'll move on and show you the definition of the "isCacheValid()" method, which, as I said before, determines the validity of the respective cache file. Please look at the following method:

private function isCacheValid(){
    // determine if cache file is valid or not
    if(file_exists($this->cacheFile)&&filemtime($this->cacheFile)
>(time()-$this->expiry)){
        return true;
    }
    return false;
}

In this case, the definition of the above method is very simple. In short, what it does is check whether the cached contents are valid or not, based on a time expiration caching trigger. If the cache file is found to be valid, the method returns true. Otherwise, the returned value will be false.

Now that you learned how some useful methods of the "TemplateProcessor" class work, go ahead and read the next section of the article. That's where I'll show you the definition of the "processTemplate()" method, which is actually the core engine of the class.