Prefix Query

More info about prefix query is in the official elasticsearch docs

Matches documents that have fields containing terms with a specified prefix.

Simple example

{
    "prefix" : { "user" : "ki" }
}

In DSL:

$prefixQuery = new PrefixQuery('user', 'ki');

$search = new Search();
$search->addQuery($prefixQuery);

$queryArray = $search->toArray();