PDO Fetch Modes
debt(d5/e1/b3/t3)
Closest to 'specialist tool catches it' (d5). The detection_hints list phpstan as the tool, and the code_pattern targets fetch(PDO::FETCH_BOTH) or fetchAll() without a fetch mode — this is a specialist static analysis tool catch, not a default linter or compiler error.
Closest to 'one-line patch or single-call swap' (e1). The quick_fix explicitly states: set PDO::ATTR_DEFAULT_FETCH_MODE => PDO::FETCH_ASSOC in the PDO constructor options array — a single configuration change.
Closest to 'localised tax' (b3). The choice affects all database queries in the codebase (web and cli contexts), but it's a configuration-level decision localised to the PDO setup. Inconsistent modes across the codebase impose a moderate maintenance tax on query code, but the rest of the system is largely unaffected.
Closest to 'minor surprise (one edge case)' (t3). The misconception is mild: developers may prefer FETCH_OBJ thinking it's better, but FETCH_ASSOC is safer. The more concrete trap is the default FETCH_BOTH wasting memory with duplicate keys — a documented gotcha but not catastrophic or architecturally contradictory.
Also Known As
TL;DR
Explanation
PDO::FETCH_ASSOC returns associative arrays (column name keys). PDO::FETCH_OBJ returns stdClass objects. PDO::FETCH_CLASS maps rows directly into a specified class. PDO::FETCH_COLUMN returns a single column as a flat array. PDO::FETCH_KEY_PAIR returns a two-column result as key→value pairs. Setting PDO::ATTR_DEFAULT_FETCH_MODE on the connection avoids repeating the constant on every fetch() call.
Common Misconception
Why It Matters
Common Mistakes
- Using the default FETCH_BOTH which returns both numeric and named keys, wasting memory.
- Forgetting to set a default fetch mode and using different modes inconsistently across the codebase.
- Using FETCH_OBJ on untrusted data where property names may conflict with magic methods.
Avoid When
- Avoid FETCH_BOTH — it duplicates every value under both numeric and named keys, wasting memory.
- Avoid FETCH_OBJ on untrusted column names — property names from user-controlled data can shadow object methods.
When To Use
- Use FETCH_ASSOC as the default for all queries — explicit, predictable, no memory waste.
- Use FETCH_CLASS when mapping query results directly to domain model objects.
- Use FETCH_KEY_PAIR when building id→name lookup maps from two-column queries.
Code Examples
// FETCH_BOTH wastes memory — every value stored twice
$row = $stmt->fetch(PDO::FETCH_BOTH); // ['id'=>1, 0=>1, 'email'=>'...', 1=>'...']$pdo->setAttribute(PDO::ATTR_DEFAULT_FETCH_MODE, PDO::FETCH_ASSOC);
// FETCH_ASSOC — safe default
$row = $stmt->fetch(); // ['id' => 1, 'email' => '...']
// FETCH_CLASS — auto-map to domain object
$stmt->setFetchMode(PDO::FETCH_CLASS, User::class);
$user = $stmt->fetch(); // User instance with properties set
// FETCH_COLUMN — flat array of one column
$emails = $stmt->fetchAll(PDO::FETCH_COLUMN, 0);
// FETCH_KEY_PAIR — id => name map
$map = $pdo->query('SELECT id, name FROM roles')->fetchAll(PDO::FETCH_KEY_PAIR);