Hola, quizas no sea la solucion mas optima, pero lo he hecho de esta manera, prueba asi:
Código PHP:
Ver original<?php
$string_json = '{"1-9" : ["1-8",{"1-7" : ["2-1","2-2","2-3","2-4"]},"1-6"]}';
echo '<br>'.$var_php['1-9']['0'];
foreach ($var_php['1-9']['1']['1-7'] as $key => $value) {
echo "<br>".$value;
}
echo '<br>'.$var_php['1-9']['2'];
Saludos