Insert into data fail after foreign key rebuilded.Mysql PHP - php

Question

I hava two table:IDCa and IDCb.At the beginning, each table has data and works fine. Table IDCb has foreign key sheetNum with IDCa's primary key sheetNum.
When I renamed IDCa to IDCabk,and drop IDCa, it worked failed.
So I runned: set foreign_key_checks=0
, drop table IDCa, create table IDCa like IDCabk.
Now IDCa is empty.After that, I found IDCb's REFERENCES table had changed from IDCa to IDCabk. So I drop IDCb foreign key ,and readded like:
alter table IDCb add constraint fk_dvcOnsh foreign key (sheetNum) references IDCa(sheetNum);
After I runned: set foreign_key_checks=1, and tried insert into data.But unfortunately, it works fail.It seems nothing wrong.
Here IDCa information, run :show create table IDCa :
|IDCa | CREATE TABLE `IDCa` (
`sheetNum` int(255) NOT NULL AUTO_INCREMENT,
.........
PRIMARY KEY (`sheetNum`)
) ENGINE=InnoDB AUTO_INCREMENT=6 DEFAULT CHARSET=utf8
Here IDCb information, run： show create table IDCb :
|IDCb | CREATE TABLE `IDCb` (
`id` int(255) NOT NULL AUTO_INCREMENT,
......
`sheetNum` int(255) DEFAULT NULL,
PRIMARY KEY (`id`),
KEY `fk_dvcOnsh` (`sheetNum`),
CONSTRAINT `fk_dvcOnsh` FOREIGN KEY (`sheetNum`) REFERENCES `IDCa` (`sheetnum`)
) ENGINE=InnoDB AUTO_INCREMENT=61 DEFAULT CHARSET=utf8
My code of inserting into data:
$conn->beginTransaction();
$query="insert into IDCa(xxx) values('aaa')";
$stmt=$conn->query($query);
$stmt->closeCursor();
$query="insert into IDCb(rrr,sheetNum) values('aaa',LAST_INSERT_ID())";
$stmt=$conn->query($query);
$stmt->closeCursor();
$conn->commit();
Show Error:
SQLSTATE[23000]: Integrity constraint violation: 1452 Cannot add or update a child row: a foreign key constraint fails (`hpc`.`IDCb`, CONSTRAINT `fk_dvcOnsh` FOREIGN KEY (`sheetnum`) REFERENCES `IDCa` (`sheetnum`))fail