What is md5() in PHP?

The md5() function computes the MD5 hash of a string and returns it as a 32-character hexadecimal number. The MD5 algorithm is widely used to verify data integrity because it produces a fixed-length string regardless of the input size.

Syntax:

md5(string, raw_output)

• string: The input string to be hashed.
• raw_output (optional): A boolean parameter that determines the output format. If set to TRUE, it returns raw binary data; otherwise, the default is FALSE, returning a 32-character hexadecimal number.

Example 1: Basic Usage of md5() in PHP

<?php
$string = "Hello, World!";
$hashed_string = md5($string);
echo $hashed_string;
?>

Output:

fc3ff98e8c6a0d3087d515c0473f8677

Example 2: Using md5() with raw output

By default, the md5() function returns a hexadecimal string. However, you can also retrieve the raw binary format by setting the second parameter to TRUE.

<?php
$string = "Hello, World!";
$hashed_string = md5($string, true);
echo bin2hex($hashed_string);  // Converts binary data to hex representation
?>

Output:

fc3ff98e8c6a0d3087d515c0473f8677

Practical Uses of md5() in PHP

1. Password Hashing (Not Recommended for Modern Security Practices): Although MD5 was once widely used for password hashing, it is now considered insecure due to vulnerabilities. For modern applications, it is recommended to use password_hash() instead.

   Example:

   $password = "securepassword";
   $password_hash = md5($password);  // Use this cautiously, prefer password_hash()
   

2. File Integrity Check: You can use md5() to check the integrity of files by comparing the MD5 hash of the original file with that of the uploaded file.

   <?php
   $original_file = file_get_contents("path_to_file.txt");
   $original_file_hash = md5($original_file);
   
   $uploaded_file = file_get_contents("path_to_uploaded_file.txt");
   $uploaded_file_hash = md5($uploaded_file);
   
   if($original_file_hash === $uploaded_file_hash) {
       echo "File is intact.";
   } else {
       echo "File has been tampered with.";
   }
   ?>
   

Key Points:

• MD5 generates a 128-bit hash (32 characters in hexadecimal).
• MD5 is not collision-resistant, meaning different inputs can sometimes produce the same hash.
• For secure password hashing, use password_hash() instead of md5().
• MD5 is commonly used for checksums, data integrity checks, and file verification.