How to add canonical tag to specific pages?

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If (this page is About-Us OR Delivery OR … OR … OR … OR …)
{
Include Canonical Tag of the type <link rel="canonical" href="http://www.example.co.uk/xxxxxxxx"/> , where xxxxxxx changes accordingly, depending on the page.
}
Else
{
Don’t include Canonical Tag
}

Code: Select all

if ($information_id == '4') {
   $this->document->addLink($this->url->link('information/information', 'information_id=6'), 'canonical');
} elseif ($information_id == '6') {
   $this->document->addLink($this->url->link('information/information', 'information_id=4'), 'canonical');
}

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$information_info = $this->model_catalog_information->getInformation($information_id);

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$this->document->addLink($this->url->link('information/information', 'information_id=6'), 'canonical');

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if ($information_id == '4') {
   $this->document->addLink('http://www.echothemes.com/extensions/blog-manager.html', 'canonical');
} elseif ($information_id == '6') {
   $this->document->addLink('http://www.echothemes.com/extensions/blog-manager.html', 'canonical');
}