how to remove map values from an array in PHP

I have this array in PHP:

Array ( [0] => Array ( [time_book_start] => 11:30 ) 
        [1] => Array ( [time_book_start] => 9:00 ) 
       )

I just need to output 11:30 and 9:00 in this manner:

['11:30','9:00']

I have used json_encode, sprintf, implode etc without success. Please help me.