I want to assign the value obtained from the concatenation of these two variables with a string.
{assign var=\"url\" value=\"{$WS_PATH}aircraft_images/{$images[i
You've used assign properly.
A simplified example could look like this:
yourphpfile.php:
$tpl = new Smarty;
$tpl->assign('var1','Hello');
$tpl->assign('var2','World');
$tpl->display('yourtemplate.tpl');
yourtemplate.tpl:
...
{assign var="url" value="{$var1} - and - {$var2}"}
{$url}
...will result to the output:
Hello - and - World