File pointer manipulation in PHP - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ Predict Output

intermediate

What is the output of this PHP code using file pointer?

Consider the following PHP code that reads a file pointer position after some operations. What will it output?

PHP

<?php
$file = fopen('php://memory', 'r+');
fwrite($file, "Hello World");
fseek($file, 6);
echo ftell($file);
fclose($file);
?>

A 6

B 11

C 0

D 5

💡 Hint

ftell() returns the current position of the file pointer after fseek().

The code writes 'Hello World' (11 characters) to the file, then moves the pointer to position 6 with fseek(). ftell() then returns 6, the current pointer position.

❓ Predict Output

intermediate

What does this PHP code output after reading with file pointer?

Given this PHP code, what will be printed?

PHP

<?php
$file = fopen('php://memory', 'r+');
fwrite($file, "abcdef");
fseek($file, 2);
echo fread($file, 3);
fclose($file);
?>

A cde

B abc

C bcd

D def

💡 Hint

fseek moves pointer to position 2, fread reads 3 bytes from there.

The string 'abcdef' is written. Pointer moves to position 2 (0-based), which is 'c'. Reading 3 bytes from there gives 'cde'.

❓ Predict Output

advanced

What is the output of this PHP code using rewind and ftell?

Analyze this PHP code snippet. What will it output?

PHP

<?php
$file = fopen('php://memory', 'r+');
fwrite($file, "12345");
fseek($file, 4);
rewind($file);
echo ftell($file);
fclose($file);
?>

A 5

B 4

C 0

D 1

💡 Hint

rewind() moves the pointer back to the start of the file.

After writing '12345', pointer moves to position 4. rewind() resets pointer to 0. ftell() then returns 0.

❓ Predict Output

advanced

What error or output does this PHP code produce?

What happens when this PHP code runs?

PHP

<?php
$file = fopen('php://memory', 'r');
fseek($file, 5);
fwrite($file, "test");
fclose($file);
?>

A Warning: fwrite() expects parameter 1 to be resource, bool given

B Warning: fwrite(): supplied resource is not writable

C No output, file pointer moved and wrote successfully

D Fatal error: fseek() expects parameter 1 to be resource

💡 Hint

The file is opened in read-only mode but fwrite is called.

Opening with 'r' mode means read-only. fwrite() on a read-only stream triggers a warning about the resource not being writable.

🧠 Conceptual

expert

How many bytes are read and what is the pointer position after this PHP code?

Consider this PHP code snippet. How many bytes does fread() read, and what is the file pointer position after reading?

PHP

<?php
$file = fopen('php://memory', 'r+');
fwrite($file, "OpenAI GPT");
fseek($file, 4);
$data = fread($file, 10);
$pos = ftell($file);
fclose($file);
echo strlen($data) . ',' . $pos;
?>

A 7,11

B 7,10

C 10,14

D 6,10

💡 Hint

fread reads up to the end of file or requested bytes, pointer moves accordingly.

The string 'OpenAI GPT' is 10 bytes. Pointer moves to 4 ('A'). Reading 10 bytes but only 6 bytes remain to EOF ('AI GPT'), so fread reads 6 bytes. Pointer moves to position 10.

Practice

1. What does the PHP function fseek() do when working with files?

easy

A. Closes the file after reading

B. Moves the file pointer to a specified position in the file

C. Reads the entire content of the file

D. Deletes the file from the system

Solution

1. Step 1: Understand the purpose of fseek()

   fseek() is used to move the file pointer to a specific position inside the file, allowing you to read or write from that point.

2. Step 2: Compare with other file functions

   Functions like reading or closing files do not move the pointer; fseek() specifically changes the pointer position.

3. Final Answer:

   Moves the file pointer to a specified position in the file -> Option B

4. Quick Check:

   fseek() moves pointer [OK]

Hint: Remember: fseek jumps pointer position in file [OK]

Common Mistakes:

• Confusing fseek() with fread() which reads data
• Thinking fseek() closes the file
• Assuming fseek() deletes the file

2. Which of the following is the correct syntax to move the file pointer to the beginning of a file in PHP?

easy

A. fseek($file, 0);

B. fseek($file, -1);

C. rewind($file, 0);

D. ftell($file, 0);

Solution

1. Step 1: Identify correct function and parameters

   To move the pointer to the start, fseek($file, 0); moves pointer to position 0 (start).

2. Step 2: Understand other functions

   rewind($file); also moves pointer to start but takes only one argument. ftell() returns position, does not move pointer.

3. Final Answer:

   fseek($file, 0); -> Option A

4. Quick Check:

   fseek with 0 moves to start [OK]

Hint: Use fseek with 0 to jump to file start [OK]

Common Mistakes:

• Using negative offset in fseek() which is invalid
• Passing extra argument to rewind()
• Confusing ftell() as pointer mover

3. Consider the following PHP code snippet:

$file = fopen('example.txt', 'r');
fseek($file, 5);
echo ftell($file);

What will be the output?

medium

A. Error

B. 0

C. EOF

D. 5

Solution

1. Step 1: Analyze fseek position

   fseek($file, 5); moves the file pointer to position 5 bytes from the start.

2. Step 2: Check ftell output

   ftell($file); returns the current position of the file pointer, which is now 5.

3. Final Answer:

   5 -> Option D

4. Quick Check:

   fseek to 5 then ftell returns 5 [OK]

Hint: ftell shows pointer after fseek moves it [OK]

Common Mistakes:

• Assuming ftell returns 0 always
• Thinking fseek returns file content
• Expecting EOF when pointer is inside file

4. What is wrong with this PHP code snippet?

$file = fopen('data.txt', 'r');
fseek($file, -10);
echo ftell($file);

medium

A. Negative offset in fseek causes error

B. fopen mode 'r' is incorrect

C. ftell cannot be used after fseek

D. File pointer is automatically at end

Solution

1. Step 1: Check fseek offset validity

   fseek() does not accept negative offsets from the start (SEEK_SET). Negative values cause an error.

2. Step 2: Validate other parts

   fopen('data.txt', 'r') is correct for reading. ftell() works after fseek(). Pointer is at start by default, not end.

3. Final Answer:

   Negative offset in fseek causes error -> Option A

4. Quick Check:

   fseek negative offset invalid [OK]

Hint: fseek offset must be zero or positive [OK]

Common Mistakes:

• Using negative offset with SEEK_SET
• Confusing fopen modes
• Thinking ftell fails after fseek

5. You want to update the 10th byte in a file without reading the whole file. Which sequence of PHP functions correctly moves the pointer and writes a character 'X' at that position?

hard

A. fopen($file, 'w'); fseek($file, 10); fwrite($file, 'X');

B. fopen($file, 'r'); rewind($file); fwrite($file, 'X');

C. fopen($file, 'r+'); fseek($file, 9); fwrite($file, 'X');

D. fopen($file, 'a'); fseek($file, 10); fwrite($file, 'X');

Solution

1. Step 1: Choose correct mode to read and write

   Mode 'r+' opens file for reading and writing without truncating it, allowing pointer movement and writing.

2. Step 2: Move pointer and write

   fseek($file, 9); moves pointer to 10th byte (index 9). fwrite($file, 'X'); writes 'X' at that position.

3. Step 3: Check other options

   Mode 'w' truncates file, 'r' alone can't write, 'a' appends at end ignoring fseek.

4. Final Answer:

   fopen($file, 'r+'); fseek($file, 9); fwrite($file, 'X'); -> Option C

5. Quick Check:

   Use 'r+' mode and fseek before fwrite [OK]

Hint: Use 'r+' mode and fseek to update file byte [OK]

Common Mistakes:

• Using 'w' mode which erases file
• Using 'a' mode which always appends
• Not moving pointer before writing