Is there any way to check the sub string is exists in main string or not. If exits, no need any another action. If not exists, I want to add some another string to main string. 

For an example:

```
$a = 'Hello world';
$b = 'Hello';

```

I want to check the `$b` is exists in `$a` or not. If exists, i am sending that variable to database. If not exists, I want to use `str_replace('Hello', $b.'World', $b)`.