how to replace all occurrences of the same values in an array

Hi i’m new to php and am just wondering how to iterate through an array

  $array = ["7","8","8","18","19","20","21","22","8"];

and replace all the values “8” with “1”

i tried

function firstFileInList($array) {
  foreach($array as $k => $v)
    if(strpos($v, "8") !== false){
      return $k;}


}

$testarray[firstFileInList($testarray)] =1;

but it only does the first instance