If you have intl extension installed, you can use Transliterator::transliterate function to create a slug easily.
<?php
$string = 'Namnet på bildtävlingen';
$slug = \Transliterator::createFromRules(
':: Any-Latin;'
. ':: NFD;'
. ':: [:Nonspacing Mark:] Remove;'
. ':: NFC;'
. ':: [:Punctuation:] Remove;'
. ':: Lower();'
. '[:Separator:] > \'-\''
)
->transliterate( $string );
echo $slug; // namnet-pa-bildtavlingen
?>