https://www.mongodb.com/zh-cn/docs/upcoming/reference/operator/aggregation/ltrim/
Use `$ltrim` in MongoDB to remove whitespace or specified characters from the beginning of a string in aggregation operations.
operator databaseltrimexpressionmanualmongodb
https://www.php.net/manual/es/function.ltrim.php
Elimina los espacios (u otros caracteres) del inicio de un string
phpltrimmanual
https://www.php.net/manual/uk/function.ltrim.php
Strip whitespace (or other characters) from the beginning of a string
phpltrimmanual