One request at a time with PHP and MySQL

Answer

Solution:

PDO's lastInsertId() won't be affected by other requests or connections so you can rely on that.

However, do you really need to store ID twice?

If the name is alwaysInv. {id} then it seems to me it makes more sense just to format that with php:Inv. <?= $row['id'] ?> or using CONCAT with SQL:select CONCAT('Inv. ', id)).

Answer

Solution:

You can only rely on last insert id if the lastInsertId() function is executed immediately following the insert statement.

Note that if the name always has the same characters followed by the identifier, then you should not save the same data in two different columns. Instead you should have a domain object with a getter for the legal name of the transaction:

class Transaction {
    private $id;

    public function getId() {
         return $this->id;
    }

    public function getLegalName() {
        return 'Inv. ' . $this->id;
    }
}
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Having said that; if you are developing a financial system, you might not actually want to reinvent the wheel. There are many PHP solutions out there, including open source ones.