why echo element only show horizontally in a div

I am doing my home woke of php in which I would like to implement many xx in a div ;

the code is like this

  echo "<div>";
  for ($j = 1; $j <= $numElement; $j++) {
           echo "<a href=#$id class='button-primary-outlined'>$j</a>";
       }

echo</di>;

i have set the width of the div but what i got is like this

all the is placed horizontally.

what happens??