fopen() / fread() / fwrite() / fclose() Since: PHP 4（2000）

// Opens a file and returns a stream resource.
$fp = fopen($filename, $mode);

// Reads the specified number of bytes.
fread($fp, $length);

// Reads one line.
fgets($fp, $length);

// Writes a string.
fwrite($fp, $string, $length);

// Checks whether the end of the file has been reached.
feof($fp);

// Closes the file.
fclose($fp);

<?php
// Open a file and read it line by line.
$fp = fopen("/var/www/html/data.txt", "r");
if ($fp === false) {
    die("Could not open the file.");
}
while (!feof($fp)) {
    $line = fgets($fp);
    if ($line !== false) {
        echo $line; // Each line is printed.
    }
}
fclose($fp);

// Read a specific number of bytes with fread().
$fp = fopen("/var/www/html/data.bin", "r");
$header = fread($fp, 4); // Read the first 4 bytes.
echo bin2hex($header); // Print the binary data as hexadecimal.
fclose($fp);

// Write to a file.
$fp = fopen("/var/www/html/output.txt", "w");
fwrite($fp, "Line 1\n");
fwrite($fp, "Line 2\n");
fclose($fp);

// Append to a log file.
$fp = fopen("/var/www/html/app.log", "a");
fwrite($fp, date("Y-m-d H:i:s") . " Process completed.\n");
fclose($fp);

// Practical example: process a CSV file line by line.
$fp = fopen("/var/www/html/users.csv", "r");
while (($line = fgets($fp)) !== false) {
    $columns = explode(",", trim($line));
    echo $columns[0] . ": " . $columns[1] . "\n"; // Split and print each line.
}
fclose($fp);

// Practical example: read a large file in chunks.
$fp = fopen("/var/www/html/large_file.dat", "r");
$chunk_size = 8192; // Read 8 KB at a time.
while (!feof($fp)) {
    $chunk = fread($fp, $chunk_size);
    // Process each chunk here.
}
fclose($fp);

Unlike file_get_contents(), which loads an entire file into memory at once, stream-based operations with fopen() let you read or write only as much data as you need at a time. When dealing with very large files, file_get_contents() can exhaust available memory because it loads the entire file at once. In such cases, using fopen() with fgets() to process one line at a time is the appropriate approach.

The write mode "w" truncates the file before writing, so any existing data will be lost. Use mode "a" if you want to append to an existing file. Always close a file with fclose() when you are done. Forgetting to close it can cause resource leaks.

For reading CSV files, PHP also provides the dedicated fgetcsv() function. Regarding file operation security, if you use user-supplied input as a file path, validate it with basename() or realpath() to prevent directory traversal attacks.