你的問題似乎用不著數(shù)組

<?php
if(!empty($_POST)){

    foreach ($_POST as $k=>$v){
        if(strstr($k, "product_")){
            $k = substr($k, strlen("product_"));
            
            echo "update product_table set value=$v where id=$k;<br />";
        }
    }
    
    exit();
}
?>

<form method="post">
    
    <?php 
        $arr= array(
            array('id'=>1,'value'=>3),
            array('id'=>2,'value'=>4),
            array('id'=>3,'value'=>5),
        );
        
        foreach($arr as $val){
            ?>
    <label>商品:<?=$val['id']?></label><input type="text" value="<?=$val['value']?>" name="product_<?=$val['id']?>" /><br> 
            <?php
        }
    ?>
                   <input type="submit" />
</form>

INSERT INTO `user_cart` (`prod_id`, `quan`)
VALUES (12, 2),
    (13, 1),
    (14, 5)
ON DUPLICATE KEY UPDATE `quan` = VALUES(`quan`);