In PHP, what is the significance of using a HEAD request to obtain a Content-MD5 for comparing file content integrity instead of relying on modification dates?

When comparing file content integrity, relying on modification dates may not be reliable as the file could be modified without changing its content. Using a HEAD request to obtain a Content-MD5 checksum allows for a more accurate comparison of file content. This checksum can be used to verify the integrity of the file by comparing it with the calculated MD5 checksum of the file's content. 

<?php

$file_url = 'https://example.com/file.txt';

// Perform a HEAD request to obtain the Content-MD5 checksum
$ch = curl_init($file_url);
curl_setopt($ch, CURLOPT_NOBODY, true);
curl_setopt($ch, CURLOPT_RETURNTRANSFER, true);
curl_setopt($ch, CURLOPT_HEADER, true);
$response = curl_exec($ch);

// Extract the Content-MD5 checksum from the response headers
preg_match('/Content-MD5: (.+)/', $response, $matches);
$content_md5 = $matches[1];

// Calculate the MD5 checksum of the file's content
$file_content = file_get_contents($file_url);
$calculated_md5 = md5($file_content);

// Compare the Content-MD5 checksum with the calculated MD5 checksum
if ($content_md5 === $calculated_md5) {
    echo 'File content integrity verified.';
} else {
    echo 'File content integrity could not be verified.';
}

curl_close($ch);

?>

Keywords

PHP HEAD request Content-MD5 file content integrity modification dates

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