<?php

define(URL_CONSTANT, '/');

function replace($article_id,$text){
    $pattern = '/\{\{(.*?)\}\}/';
    $replacement = '<img src="'.URL_CONSTANT.'articles/'.$article_id.'/'.'${1}'.'"></img>';
    return preg_replace($pattern, $replacement, $text);
}

echo replace(42, 'hello {{something.jpg}}');

hello <img src="/articles/42/something.jpg"></img>