CSV file reading and writing in PHP - Step-by-Step Execution

Concept Flow - CSV file reading and writing

Open CSV file

↓

Read line

↓

Parse CSV line

↓

Process data

↓

Repeat until EOF

↓

Close file

↓

Open CSV file for writing

↓

Write data line

↓

Repeat for all lines

↓

Close file

Open a CSV file, read each line parsing CSV data, process it, then close. For writing, open file, write lines, then close.

Execution Sample

PHP

<?php
$handle = fopen('data.csv', 'r');
while (($data = fgetcsv($handle)) !== false) {
    print_r($data);
}
fclose($handle);
?>

This code reads a CSV file line by line and prints each row as an array.

Execution Table

Step	Action	Function Call	Data Read/Write	Output/Result
1	Open file for reading	fopen('data.csv', 'r')	File opened	Resource handle assigned
2	Read first line	fgetcsv(handle)	"John,Doe,30"	Array('John', 'Doe', '30') printed
3	Read second line	fgetcsv(handle)	"Jane,Smith,25"	Array('Jane', 'Smith', '25') printed
4	Read third line	fgetcsv(handle)	"Bob,Brown,40"	Array('Bob', 'Brown', '40') printed
5	Read next line	fgetcsv(handle)	EOF reached	false returned, loop ends
6	Close file	fclose(handle)	File closed	No output
7	Open file for writing	fopen('output.csv', 'w')	File opened	Resource handle assigned
8	Write first line	fputcsv(handle, ['Name', 'Age'])	Writes "Name,Age\n"	1 (success) returned
9	Write second line	fputcsv(handle, ['Alice', '28'])	Writes "Alice,28\n"	1 (success) returned
10	Close file	fclose(handle)	File closed	No output

💡 Reading stops when fgetcsv returns false at EOF; writing stops after all lines written and file closed.

Variable Tracker

Variable	Start	After Step 2	After Step 3	After Step 4	After Step 5	After Step 6	After Step 8	After Step 9	Final
handle	null	resource	resource	resource	resource	null	resource	resource	null
data	null	['John', 'Doe', '30']	['Jane', 'Smith', '25']	['Bob', 'Brown', '40']	false	false	null	null	null

Key Moments - 3 Insights

Why does the loop stop reading the CSV file?

The loop stops because fgetcsv returns false when it reaches the end of the file, as shown in step 5 of the execution_table.

What does fputcsv return after writing a line?

fputcsv returns 1 on success after writing a line, as seen in steps 8 and 9 of the execution_table.

Why do we need to close the file after reading or writing?

Closing the file releases system resources and ensures data is saved properly, shown in steps 6 and 10 where fclose is called.

Visual Quiz - 3 Questions

Look at the execution_table, what is the value of 'data' after step 3?

A ['Jane', 'Smith', '25']

B ['John', 'Doe', '30']

C false

D null

💡 Hint

Check the 'data' column in the variable_tracker after step 3.

At which step does reading from the CSV file stop?

A Step 4

B Step 5

C Step 6

D Step 7

💡 Hint

Look at the execution_table where fgetcsv returns false indicating EOF.

If we forget to call fclose after writing, what might happen?

A Data might not be saved properly

B File will be deleted automatically

C Reading will stop early

D Nothing, fclose is optional

💡 Hint

Refer to key_moments about why closing files is important.

Concept Snapshot

CSV file reading and writing in PHP:
- Use fopen() to open file ('r' for read, 'w' for write)
- Use fgetcsv() to read CSV lines as arrays
- Use fputcsv() to write arrays as CSV lines
- Loop reading until fgetcsv returns false (EOF)
- Always fclose() to close the file and save resources

Full Transcript

This visual execution trace shows how to read and write CSV files in PHP. First, fopen opens the file for reading. Then, a while loop uses fgetcsv to read each line as an array until it returns false at the end of the file. Each array is printed. After reading, fclose closes the file. For writing, fopen opens the file in write mode. fputcsv writes arrays as CSV lines. After writing all lines, fclose closes the file. Variables like 'handle' track the file resource, and 'data' holds the current CSV row. Closing files is important to save data and free resources.

Practice

1. Which PHP function is used to read a line from a CSV file and parse it into an array?

easy

A. fgetcsv()

B. fputcsv()

C. fread()

D. fwrite()

Solution

1. Step 1: Understand CSV reading functions

   PHP provides fgetcsv() to read a line from a CSV file and convert it into an array of values.

2. Step 2: Differentiate from writing functions

   fputcsv() writes arrays to CSV files, while fread() and fwrite() handle raw file reading/writing without CSV parsing.

3. Final Answer:

   fgetcsv() -> Option A

4. Quick Check:

   Read CSV line as array = fgetcsv() [OK]

Hint: Use fgetcsv() to read CSV lines into arrays [OK]

Common Mistakes:

• Confusing fgetcsv() with fputcsv()
• Using fread() which reads raw data, not CSV parsed
• Trying to read CSV without parsing into arrays

2. Which of the following is the correct way to open a CSV file for writing in PHP?

easy

A. $file = fopen('data.csv', 'r');

B. $file = fopen('data.csv', 'rw');

C. $file = fopen('data.csv', 'w');

D. $file = fopen('data.csv', 'x');

Solution

1. Step 1: Understand fopen modes

   Mode 'w' opens a file for writing and truncates it if it exists, perfect for writing CSV files.

2. Step 2: Check other modes

   'r' is read-only, 'rw' is invalid, 'x' creates a new file but fails if it exists.

3. Final Answer:

   $file = fopen('data.csv', 'w'); -> Option C

4. Quick Check:

   Open file for writing = 'w' mode [OK]

Hint: Use 'w' mode in fopen() to write files [OK]

Common Mistakes:

• Using 'r' mode when intending to write
• Using invalid mode 'rw'
• Using 'x' which fails if file exists

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

$file = fopen('data.csv', 'r');
$data = fgetcsv($file);
fclose($file);
print_r($data);

Assuming the first line of data.csv is:
John,Doe,30

medium

A. Array ( [0] => John [1] => Doe [2] => 30 )

B. John,Doe,30

C. Array ( [0] => 'John,Doe,30' )

D. Error: Cannot read file

Solution

1. Step 1: Understand fgetcsv() output

   fgetcsv() reads one CSV line and returns an array splitting values by commas.

2. Step 2: Analyze given CSV line

   The line 'John,Doe,30' splits into array elements: 'John', 'Doe', and '30'.

3. Final Answer:

   Array ( [0] => John [1] => Doe [2] => 30 ) -> Option A

4. Quick Check:

   fgetcsv() splits CSV line into array [OK]

Hint: fgetcsv() returns array of CSV fields from one line [OK]

Common Mistakes:

• Expecting a string instead of an array
• Assuming fgetcsv returns raw line string
• Not closing the file after reading

4. Identify the error in the following PHP code that writes data to a CSV file:

$file = fopen('output.csv', 'r');
fputcsv($file, ['Alice', 'Smith', 25]);
fclose($file);

medium

A. Array syntax is incorrect

B. fputcsv() cannot write arrays

C. Missing fclose() call

D. File opened in read mode instead of write mode

Solution

1. Step 1: Check fopen mode

   The file is opened with 'r' (read) mode, which does not allow writing.

2. Step 2: Understand fputcsv requirements

   fputcsv() requires the file to be opened in write ('w') or append ('a') mode to write data.

3. Final Answer:

   File opened in read mode instead of write mode -> Option D

4. Quick Check:

   Open file in write mode before writing [OK]

Hint: Open file with 'w' or 'a' before writing CSV [OK]

Common Mistakes:

• Opening file in 'r' mode when writing
• Forgetting to close the file
• Misunderstanding fputcsv() input

5. You want to read a CSV file and write only rows where the third column is greater than 50 to a new CSV file. Which code snippet correctly implements this?

hard

A. $in = fopen('input.csv', 'w');
$out = fopen('output.csv', 'r');
while (($row = fgetcsv($in)) !== false) {
if ($row[2] > 50) {
fputcsv($out, $row);
}
}
fclose($in);
fclose($out);

B. $in = fopen('input.csv', 'r');
$out = fopen('output.csv', 'w');
while (($row = fgetcsv($in)) !== false) {
if ($row[2] > 50) {
fputcsv($out, $row);
}
}
fclose($in);
fclose($out);

C. $in = fopen('input.csv', 'r');
$out = fopen('output.csv', 'w');
while (($row = fgetcsv($out)) !== false) {
if ($row[2] > 50) {
fputcsv($in, $row);
}
}
fclose($in);
fclose($out);

D. $in = fopen('input.csv', 'r');
$out = fopen('output.csv', 'w');
while (($row = fgets($in)) !== false) {
if ($row[2] > 50) {
fputcsv($out, $row);
}
}
fclose($in);
fclose($out);

Solution

1. Step 1: Open input file for reading and output file for writing

   Use 'r' mode for reading input and 'w' mode for writing output.

2. Step 2: Read each CSV row with fgetcsv() from input and check third column

   Use fgetcsv($in) to read rows, then check if $row[2] > 50.

3. Step 3: Write qualifying rows to output with fputcsv()

   If condition passes, write row to output file using fputcsv($out, $row).

4. Final Answer:

   Code snippet B correctly reads, filters, and writes CSV rows -> Option B

5. Quick Check:

   Read with fgetcsv(), filter by column, write with fputcsv() [OK]

Hint: Read with fgetcsv(), filter rows, write with fputcsv() [OK]

Common Mistakes:

• Opening files with wrong modes
• Using fgets() instead of fgetcsv() for CSV reading
• Mixing input and output file handles