How to convert a string from a worksheet to date format for mysql?

I'm looking for a way to do this, but I'm not sure how to do it.

try converting to mysql but in the insert the following column error appears

  

'column name' can not be null

follow the insert

$salesplan_insert = mysqli_query($link,"insert into prom.absenteismo('matricula' , 'funcionario' , 'lider' , 'linha' , 'falta', 'data_falta', 'observacao') values ('$a','$b','$c','$d','$e', date_format('$f', '%Y-%m-%d'),'$g')");

I also used STR_TO_DATE() also did not work does anyone know how to do this?