flawopen.com/SQL Injection in PHP/mysqli vs PDO
Neither is inherently safer — both are equally safe when used with prepared statements and bound parameters. The safety comes from parameterization, not from which library you pick.
This is a portability distinction, not a security one — PDO's driver abstraction doesn't add extra safety over mysqli's prepared statements.
PDO lets you write :id instead of just ?, which can make complex queries more readable — a readability advantage, not a safety one, since positional placeholders in mysqli are equally safe when used correctly.
Building a query string with concatenation and passing it to either library's non-prepared execute method (mysqli_query() or PDO::query() without parameters) is equally vulnerable in both — the library doesn't protect you if you don't use its parameterized API.
Pick based on whether you need multi-database portability (PDO) or you're MySQL-only and want mysqli's MySQL-specific features — not based on security, since both are safe when used correctly and both are exactly as vulnerable when misused.