Fetch modes and styles in PHP - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ Predict Output

intermediate

What is the output of this PHP PDO fetch mode code?

Consider the following PHP code snippet using PDO to fetch data from a database. What will be the output of the print_r statement?

PHP

<?php
$pdo = new PDO('sqlite::memory:');
$pdo->exec("CREATE TABLE users (id INTEGER, name TEXT);");
$pdo->exec("INSERT INTO users VALUES (1, 'Alice'), (2, 'Bob');");
$stmt = $pdo->query("SELECT * FROM users WHERE id = 1");
$result = $stmt->fetch(PDO::FETCH_ASSOC);
print_r($result);
?>

A Array ( [id] => 1 [name] => Alice )

B Array ( [0] => 1 [1] => Alice )

C Array ( [id] => 1 [0] => 1 [name] => Alice [1] => Alice )

D NULL

💡 Hint

PDO::FETCH_ASSOC returns an associative array indexed by column names.

The fetch mode PDO::FETCH_ASSOC returns an array indexed by column names only, so the output contains keys 'id' and 'name' with their respective values.

❓ Predict Output

intermediate

What does this PHP PDO fetch style code output?

Given this PHP code using PDO, what will be the output of the print_r statement?

PHP

<?php
$pdo = new PDO('sqlite::memory:');
$pdo->exec("CREATE TABLE products (id INTEGER, name TEXT);");
$pdo->exec("INSERT INTO products VALUES (10, 'Pen'), (20, 'Pencil');");
$stmt = $pdo->query("SELECT * FROM products WHERE id = 20");
$result = $stmt->fetch(PDO::FETCH_NUM);
print_r($result);
?>

A Array ( [0] => 20 [1] => Pencil )

B Array ( [id] => 20 [name] => Pencil )

C Array ( [id] => 20 [0] => 20 [name] => Pencil [1] => Pencil )

D false

💡 Hint

PDO::FETCH_NUM returns a numeric array indexed by column numbers.

The fetch mode PDO::FETCH_NUM returns an array indexed by column numbers starting at 0, so the output contains keys 0 and 1 with values 20 and 'Pencil'.

❓ Predict Output

advanced

What is the output of this PHP PDO fetch with FETCH_OBJ?

Look at this PHP code using PDO with fetch mode FETCH_OBJ. What will be the output of the echo statement?

PHP

<?php
$pdo = new PDO('sqlite::memory:');
$pdo->exec("CREATE TABLE employees (id INTEGER, name TEXT);");
$pdo->exec("INSERT INTO employees VALUES (5, 'John');");
$stmt = $pdo->query("SELECT * FROM employees WHERE id = 5");
$result = $stmt->fetch(PDO::FETCH_OBJ);
echo $result->name;
?>

A Array

B John

C 5

D Notice: Trying to get property of non-object

💡 Hint

PDO::FETCH_OBJ returns an anonymous object with properties named after columns.

Using PDO::FETCH_OBJ returns an object where column names are properties. Accessing $result->name prints 'John'.

❓ Predict Output

advanced

What error does this PHP PDO fetch code produce?

What error will this PHP code produce when executed?

PHP

<?php
$pdo = new PDO('sqlite::memory:');
$pdo->exec("CREATE TABLE items (id INTEGER, name TEXT);");
$pdo->exec("INSERT INTO items VALUES (1, 'Book');");
$stmt = $pdo->query("SELECT * FROM items WHERE id = 1");
$result = $stmt->fetch(PDO::FETCH_CLASS);
echo $result->name;
?>

A Fatal error: Uncaught ArgumentCountError: PDO::fetch() expects at least 1 parameter

B Warning: PDO::fetch() expects parameter 1 to be int, string given

C Book

D Fatal error: Class name must be specified when using PDO::FETCH_CLASS

💡 Hint

PDO::FETCH_CLASS requires a class name as the second argument.

When using PDO::FETCH_CLASS, you must specify the class name as the second argument to fetch(). Omitting it causes a fatal error.

🧠 Conceptual

expert

Which fetch mode returns both numeric and associative keys?

In PHP PDO, which fetch mode returns an array containing both numeric indexes and associative keys for the columns?

A PDO::FETCH_NUM

B PDO::FETCH_ASSOC

C PDO::FETCH_BOTH

D PDO::FETCH_OBJ

💡 Hint

This fetch mode combines numeric and associative keys in the result array.

PDO::FETCH_BOTH returns an array indexed by both column names and column numbers, so you get both numeric and associative keys.

Practice

1. Which PHP PDO fetch mode returns each row as an associative array with column names as keys?

easy

A. PDO::FETCH_OBJ

B. PDO::FETCH_NUM

C. PDO::FETCH_ASSOC

D. PDO::FETCH_COLUMN

Solution

1. Step 1: Understand fetch modes

   PDO::FETCH_ASSOC returns rows as associative arrays with column names as keys.

2. Step 2: Compare with other modes

   PDO::FETCH_NUM returns numeric arrays, PDO::FETCH_OBJ returns objects, PDO::FETCH_COLUMN returns a single column value.

3. Final Answer:

   PDO::FETCH_ASSOC -> Option C

4. Quick Check:

   Associative array = PDO::FETCH_ASSOC [OK]

Hint: Associative arrays use PDO::FETCH_ASSOC for column-name keys [OK]

Common Mistakes:

• Confusing PDO::FETCH_NUM with associative arrays
• Using PDO::FETCH_OBJ when array is needed
• Thinking PDO::FETCH_COLUMN returns full rows

2. Which of the following is the correct syntax to fetch a single row as an object using PDO?

easy

A. $stmt->fetch(PDO::FETCH_OBJ);

B. $stmt->fetch(PDO::FETCH_ASSOC);

C. $stmt->fetchAll(PDO::FETCH_OBJ);

D. $stmt->fetchColumn();

Solution

1. Step 1: Identify fetch method for single row as object

   fetch() returns one row; PDO::FETCH_OBJ returns it as an object.

2. Step 2: Check other options

   fetchAll() returns all rows, fetchColumn() returns a single column value, fetch(PDO::FETCH_ASSOC) returns an associative array.

3. Final Answer:

   $stmt->fetch(PDO::FETCH_OBJ); -> Option A

4. Quick Check:

   Single row object = fetch(PDO::FETCH_OBJ) [OK]

Hint: Use fetch(PDO::FETCH_OBJ) for one row as object [OK]

Common Mistakes:

• Using fetchAll() when only one row is needed
• Using fetchColumn() to get full row
• Confusing FETCH_ASSOC with FETCH_OBJ

3. What will be the output of the following PHP code?

$stmt = $pdo->query('SELECT id, name FROM users LIMIT 1');
$row = $stmt->fetch(PDO::FETCH_NUM);
echo $row[0] . ', ' . $row[1];

medium

A. id, name

B. 1, Alice

C. Array, Array

D. Error: Undefined index

Solution

1. Step 1: Understand PDO::FETCH_NUM

   fetch(PDO::FETCH_NUM) returns a numeric array with column values indexed 0,1,...

2. Step 2: Analyze echo statement

   $row[0] is the first column (id), $row[1] is the second column (name). Assuming first user has id=1 and name='Alice', output is '1, Alice'.

3. Final Answer:

   1, Alice -> Option B

4. Quick Check:

   Numeric array indexes = 0,1 values [OK]

Hint: FETCH_NUM returns numeric keys, use indexes 0,1 for columns [OK]

Common Mistakes:

• Expecting column names as keys with FETCH_NUM
• Using associative keys with numeric array
• Assuming fetchColumn returns full row

4. Identify the error in this PHP code snippet that fetches all user names as a single column array:

$stmt = $pdo->query('SELECT name FROM users');
$names = $stmt->fetch(PDO::FETCH_COLUMN);
print_r($names);

medium

A. print_r cannot print arrays

B. PDO::FETCH_COLUMN is invalid fetch mode

C. Query should select id, not name

D. fetch() returns only one value, should use fetchAll()

Solution

1. Step 1: Understand fetch() vs fetchAll()

   fetch() returns a single row or value, fetchAll() returns all rows.

2. Step 2: Check usage of FETCH_COLUMN

   FETCH_COLUMN fetches a single column value per row; to get all names, fetchAll(PDO::FETCH_COLUMN) is needed.

3. Final Answer:

   fetch() returns only one value, should use fetchAll() -> Option D

4. Quick Check:

   fetchAll(PDO::FETCH_COLUMN) gets all column values [OK]

Hint: Use fetchAll(PDO::FETCH_COLUMN) to get all values of one column [OK]

Common Mistakes:

• Using fetch() instead of fetchAll() for multiple rows
• Thinking FETCH_COLUMN fetches all rows automatically
• Misunderstanding print_r usage

5. You want to fetch all rows from a database as objects but only include rows where the 'active' column is true. Which PHP code snippet correctly applies this filter using fetch modes and styles?

hard

A. $rows = $stmt->fetchAll(PDO::FETCH_OBJ); $activeRows = array_filter($rows, fn($r) => $r->active);

B. $rows = $stmt->fetchAll(PDO::FETCH_ASSOC); foreach ($rows as $r) { if ($r['active']) echo $r; }

C. $rows = $stmt->fetchAll(PDO::FETCH_COLUMN); $activeRows = array_filter($rows, fn($r) => $r->active);

D. $rows = $stmt->fetch(PDO::FETCH_OBJ); while ($rows->active) { echo $rows; }

Solution

1. Step 1: Fetch all rows as objects

   fetchAll(PDO::FETCH_OBJ) returns an array of objects, each representing a row.

2. Step 2: Filter rows where 'active' is true

   array_filter with a callback checks the 'active' property on each object, keeping only those with true.

3. Step 3: Check other options

   $rows = $stmt->fetchAll(PDO::FETCH_ASSOC); foreach ($rows as $r) { if ($r['active']) echo $r; } uses associative arrays but does not collect filtered rows properly; $rows = $stmt->fetchAll(PDO::FETCH_COLUMN); $activeRows = array_filter($rows, fn($r) => $r->active); uses FETCH_COLUMN which returns single column values, not objects; $rows = $stmt->fetch(PDO::FETCH_OBJ); while ($rows->active) { echo $rows; } misuses fetch() and while loop.

4. Final Answer:

   $rows = $stmt->fetchAll(PDO::FETCH_OBJ); $activeRows = array_filter($rows, fn($r) => $r->active); -> Option A

5. Quick Check:

   fetchAll(PDO::FETCH_OBJ) + array_filter for property check [OK]

Hint: Use fetchAll(PDO::FETCH_OBJ) then array_filter by property [OK]

Common Mistakes:

• Using fetch() instead of fetchAll() for multiple rows
• Using FETCH_COLUMN when objects are needed
• Trying to filter before fetching all rows