Reading files (fread, fgets, file) in PHP - Cheat Sheet & Quick Revision

Recall & Review

beginner

What does the fread function do in PHP?

fread reads a specified number of bytes from an open file pointer. It is useful for reading large files in chunks.

beginner

How does fgets differ from fread?

fgets reads a single line from a file pointer, stopping at a newline or the specified length, while fread reads a fixed number of bytes regardless of lines.

beginner

What is the purpose of the file function in PHP?

The file function reads an entire file into an array, where each element is a line from the file. It is a quick way to get all lines at once.

beginner

What must you do before using fread or fgets?

You must first open the file using fopen to get a file pointer resource.

beginner

Why should you always close a file after reading it?

Closing a file with fclose frees system resources and avoids potential file locks or memory leaks.

Which function reads a whole file into an array, with each line as an element?

A fread()

B file()

C fgets()

D fopen()

file() reads the entire file into an array of lines.

What does fgets() read from a file?

A A single line

B The entire file

C A fixed number of bytes

D File metadata

fgets() reads one line at a time from a file pointer.

Before using fread(), what must you do?

A Close the file

B Nothing, just call <code>fread()</code>

C Use <code>file()</code>

D Open the file with <code>fopen()</code>

You must open the file first with fopen() to get a file pointer.

What is the main reason to call fclose() after reading a file?

A To write to the file

B To read the file again

C To free system resources

D To rename the file

fclose() frees system resources and closes the file properly.

Which function reads a specific number of bytes from a file?

A fread()

B file()

C fopen()

D fgets()

fread() reads a set number of bytes from a file pointer.

Explain how to read a file line by line in PHP using fgets.

Think about opening the file, reading lines until no more lines, then closing.

Describe the difference between fread and file functions for reading files.

Consider how much data each reads and how they return it.

Practice

1. Which PHP function reads a file line by line, returning one line at a time?

easy

A. fopen()

B. fread()

C. file()

D. fgets()

Solution

1. Step 1: Understand function behavior

   fgets() reads a file one line at a time, useful for line-by-line processing.

2. Step 2: Compare with other functions

   fread() reads bytes, file() reads whole file into array, fopen() opens file but does not read.

3. Final Answer:

   fgets() -> Option D

4. Quick Check:

   Line-by-line reading = fgets() [OK]

Hint: fgets() reads one line at a time from a file [OK]

Common Mistakes:

• Confusing fread() with fgets()
• Thinking file() reads line by line
• Using fopen() to read data directly

2. Which of the following is the correct syntax to read 100 bytes from a file pointer $fp using fread()?

easy

A. fread($fp);

B. fread(100, $fp);

C. fread($fp, 100);

D. fread(100);

Solution

1. Step 1: Recall fread() parameter order

   fread() takes the file pointer first, then the number of bytes to read.

2. Step 2: Validate syntax correctness

   Only fread($fp, 100); matches the correct order and required parameters.

3. Final Answer:

   fread($fp, 100); -> Option C

4. Quick Check:

   fread(fp, bytes) syntax = fread($fp, 100); [OK]

Hint: fread() needs file pointer first, then byte count [OK]

Common Mistakes:

• Swapping parameters order
• Omitting byte count
• Passing only one argument

3. What will be the output of the following PHP code if the file data.txt contains:

Line1
Line2
Line3

Code:

$lines = file('data.txt');
echo trim($lines[1]);

medium

A. Line2

B. Line1

C. Line3

D. Error

Solution

1. Step 1: Understand file() output

   file() reads the whole file into an array, each element is a line including newline.

2. Step 2: Access second line and trim

   $lines[1] is the second line "Line2\n"; trim() removes newline, so output is "Line2".

3. Final Answer:

   Line2 -> Option A

4. Quick Check:

   file()[1] trimmed = Line2 [OK]

Hint: file() returns array of lines, index 1 is second line [OK]

Common Mistakes:

• Confusing array index starting at 0
• Not trimming newline characters
• Expecting fread() output instead

4. Identify the error in this PHP code snippet that tries to read the first 10 bytes of a file:

$fp = fopen('file.txt', 'r');
$content = fread(10, $fp);
echo $content;

medium

A. fopen() mode should be 'rb' instead of 'r'

B. Parameters of fread() are reversed

C. fread() requires a third parameter

D. fopen() returns false, file not opened

Solution

1. Step 1: Check fread() parameter order

   fread() expects file pointer first, then number of bytes. Here they are reversed.

2. Step 2: Confirm other parts

   fopen('file.txt', 'r') is valid for reading text files; no third parameter needed; no info about fopen failure.

3. Final Answer:

   Parameters of fread() are reversed -> Option B

4. Quick Check:

   fread(fp, bytes) order matters [OK]

Hint: fread() needs file pointer first, then bytes [OK]

Common Mistakes:

• Swapping fread() arguments
• Assuming fopen() mode error without evidence
• Expecting extra fread() parameters

5. You want to read a text file and print only the lines that contain the word "error" (case-insensitive). Which approach is best in PHP?

hard

A. Use fgets() in a loop, check each line with stripos()

B. Use fread() to read entire file as bytes, then search for "error"

C. Use file() to read all lines, then loop and check each line with stripos()

D. Use fopen() and print file pointer directly

Solution

1. Step 1: Consider memory and line processing

   fgets() reads line by line, efficient for large files and allows checking each line.

2. Step 2: Use stripos() for case-insensitive search

   Inside the loop, stripos() finds "error" ignoring case, so only matching lines are printed.

3. Final Answer:

   Use fgets() in a loop, check each line with stripos() -> Option A

4. Quick Check:

   Line-by-line read + case-insensitive check = fgets() + stripos() [OK]

Hint: Use fgets() loop with stripos() for line filtering [OK]

Common Mistakes:

• Using fread() for line filtering
• Reading whole file into memory unnecessarily
• Printing file pointer instead of content