PDO: Integrity constraint violation (UPDATE TABLE) - php

Question

First Table:
CREATE TABLE Portfolio_Categories
(
cat_id int(11) NOT NULL,
cat_title varchar(255) NOT NULL,
cat_dir varchar(255) NOT NULL
)
ENGINE=InnoDB AUTO_INCREMENT=9 DEFAULT CHARSET=latin1;
Second Table:
CREATE TABLE Portfolio_Images
(
img_id int(11) NOT NULL,
cat_id int(11) NOT NULL,
img varchar(255) NOT NULL,
img_title varchar(255) DEFAULT NULL
)
ENGINE=InnoDB AUTO_INCREMENT=21 DEFAULT CHARSET=latin1 COMMENT='Table to store the Portfolio Images';
Constraints:
ALTER TABLE Portfolio_Categories
ADD PRIMARY KEY (cat_id);
ALTER TABLE Portfolio_Images
ADD PRIMARY KEY (img_id), ADD KEY cat_id (cat_id), ADD KEY img_id (img_id);
ALTER TABLE Portfolio_Images ADD CONSTRAINT cat_id FOREIGN KEY
(cat_id) REFERENCES Portfolio_Categories (cat_id);
My PHP Code:
$query = "UPDATE
Portfolio_Images
SET
Portfolio_Images.img = :new_img,
Portfolio_Images.img_title = :new_tit,
Portfolio_Images.cat_id =
(SELECT t_cat.cat_id FROM (SELECT * FROM Portfolio_Categories) AS t_cat WHERE t_cat.cat_title = :new_cat)
WHERE
Portfolio_Images.img = :old_img;";
$stmt = $_MySQLConn->prepare($query);
$stmt->bindParam(':new_img', $new_img);
$stmt->bindParam(':new_tit', $new_tit);
$stmt->bindParam(':new_cat', $new_cat);
$stmt->bindParam(':old_img', $old_img);
if($stmt->execute())
{
$return_value = array('success'=>true,
'new_img:'=>$new_img,
'new_tit'=>$new_tit,
'new_cat'=>$new_cat,
'old_img'=>$old_img);
}
else
{
$return_value = array('success'=>false,'error_code'=>'Could not execute query');
}
What should it do:
It should update my table without any error (as it does if I run the statement directly)
What does it do:
PHP shows me this error message:
SQLSTATE[23000]: Integrity constraint violation: 1452 Cannot add or
update a child row: a foreign key constraint fails
(1_new.Portfolio_Images, CONSTRAINT cat_id FOREIGN KEY
(cat_id) REFERENCES Portfolio_Categories (cat_id))

Answer 1

This means that you are inserting a row in Portfolio_Images where the value in Portfolio_Images.cat_id does not exist in Portfolio_Categories.cat_id.
In other words, if in Portfolio_Categories.cat_id you only have the values 1, 2, 4 you can only insert in Portfolio_Images.cat_id the values 1, 2 or 4.