This matches with any fieldName that includes the string:
$mongoRegex = new MongoDB\BSON\Regex("$string", "i");
This matches with any fieldName that STARTS with the string:
$mongoRegex = new MongoDB\BSON\Regex("^$string", "i");
$cursor = $collection->find( [ 'fieldName' => $mongoRegex ] );
$docs = [];
foreach($cursor as $doc){
$docs[] = $doc;
}
return $docs;