// the VARiables
$array = array("a","b","c","d");
$desiredLength = 71;
$newArray = array();
// create a new array wITh AT LEAST the desired number of elements by joining the array at the end of the new array
while(count($newArray) <= $desiredLength){
    $newArray = array_merge($newArray,$array);
}
// reduce the new array to the desired length (as there might be too many elements in the new array
$array = array_slice($newArray,$desiredLength);