Regular expression:
pattern: (/\*\*)\n\s*\*\s*(\@var.*)\n\s*(\*/)
replacement: $1 $2 $3
It will convert this:
/**
* @var string
/ *
into one liner:
/** @var string */
In your IDE (like PHPStorm) select lines you want to convert, press Crl+R (Find and Replace), in the dialog select Regex and In Selection, enter pattern into search box and replacement into replace box and click Replace all