How to insert multiple MySQL table rows from a PHP array

$categories = array(1,2,3,4);
$bookId = 5;

foreach ($categories as $categoryId) {

    $data = array(
        'book_id' => (int)$bookId,
        'category_id' => (int)$categoryId
    );

    $query = "INSERT INTO `lookup` SET ";
    $fields = array();
    foreach ($data as $field => $value) {
        $fields[] = "`$field` = '$value'";
    }
    $fields = implode(', ', $fields);
    $query .= $fields;

    mysql_query($query);
}

You can build up an insert using implode to insert many rows. Something like this:-

<?php

$Category = 123;

$Books = array(1,2,3,4,5,6,7,8,9,10);

$sql = "INSERT INTO book_category_link(NULL, category_id, book_id)
        VALUES ($Category".implode("),($category", $Books).")";

?>

Or if you want to use the existing tables to get the ids then something like this:-

<?php

$Category = (123, 456, 789);

$Books = array(1,2,3,4,5,6,7,8,9,10);

$sql = "INSERT INTO book_category_link(NULL, category_id, book_id)
        SELECT NULL, categories.id, books.id
        FROM books
        CROSS JOIN categories
        WHERE books.id IN (".implode(",", $Books).")
        AND categories.id IN (".implode(",", $Category).")";

?>

It's really simple. First you insert the the book into the books table and then get the ID of it. Let's say for this example it is the ID 5.

Lets say you have the following categorie id's from the checkboxes: array(21, 42, 64); Now you can construct a query like this to insert them into the lookup table:

INSERT INTO `lookup` (book_id, category_id') VALUES (5, 21), (5, 42), (5, 64);

No, you can't insert into multiple tables in one MySQL command. You can however use transactions.

INSERT ...
SELECT LAST_INSERT_ID() INTO @mysql_variable_here;
INSERT INTO table2 (@mysql_variable_here, ...);
INSERT INTO table3 (@mysql_variable_here, ...);

if you want to add multiple rows inserted then u can use the syntax

INSERT INTO  table (col1, col2) VALUES ('value', 'value'), (value, 'value');

The very basic idea is to iterate a loop on the categories submitted and run an insert query containing category ID and book ID into the lookup table.

$bookID= $_POST['bookid'];
foreach ($categories as $category) {

    mysql_query("INSERT INTO lookup(bookid,categoryid) values($bookID,$catetory)");
}