file_get_contents() / file_put_contents() Since: PHP 4（2000）

// Reads an entire file as a string.
file_get_contents($filename, $use_include_path, $context, $offset, $length);

// Writes a string to a file.
file_put_contents($filename, $data, $flags, $context);

// Reads an entire file as an array of lines.
file($filename, $flags, $context);

file_get_contents() returns the file contents as a string, or false on failure. file_put_contents() returns the number of bytes written, or false on failure. file() returns an array of lines.

<?php
// Read the entire file as a string.
$content = file_get_contents("/var/www/html/data.txt");
echo $content; // Outputs the raw file contents.

// Error handling when the file cannot be read.
$content = file_get_contents("/var/www/html/not_found.txt");
if ($content === false) {
    echo "Failed to read the file.";
}

// Read a JSON file and decode it into an associative array.
$json = file_get_contents("/var/www/html/config.json");
$config = json_decode($json, true);
echo $config["site_name"]; // Outputs the value for the given JSON key.

// Write a string to a file.
$data = "Log entry: " . date("Y-m-d H:i:s");
file_put_contents("/var/www/html/log.txt", $data);

// Append to a file using the FILE_APPEND flag.
$log = date("Y-m-d H:i:s") . " Access received.\n";
file_put_contents("/var/www/html/access.log", $log, FILE_APPEND);

// Write with an exclusive lock using the LOCK_EX flag.
file_put_contents("/var/www/html/counter.txt", "100", LOCK_EX);

// Use FILE_APPEND and LOCK_EX together.
file_put_contents("/var/www/html/access.log", $log, FILE_APPEND | LOCK_EX);

// Read a file as an array of lines using file().
$lines = file("/var/www/html/data.txt", FILE_IGNORE_NEW_LINES | FILE_SKIP_EMPTY_LINES);
foreach ($lines as $line) {
    echo $line . "\n"; // Outputs each non-empty line.
}

// Practical example: process CSV data with file().
$lines = file("/var/www/html/users.csv", FILE_IGNORE_NEW_LINES);
foreach ($lines as $line) {
    $columns = explode(",", $line);
    echo $columns[0] . ": " . $columns[1] . "\n"; // Outputs columns 1 and 2 of each row.
}

file_get_contents() and file_put_contents() are convenient functions that open, read or write, and close a file in a single call. Compared to stream-based operations using fopen() / fread() / fclose(), the code is much more concise, making these functions the go-to choice when working with an entire file at once.

file_put_contents() overwrites the file contents by default. To append instead, pass FILE_APPEND as the third argument. If multiple processes may write to the file simultaneously, use LOCK_EX to acquire an exclusive lock and prevent data corruption.

file_get_contents() can also send HTTP requests by accepting a URL, but if you need fine-grained control over headers or timeouts for API access, consider using cURL instead. To check whether a file exists before reading it, use file_exists().