PHP : Powerful encrypt and decrypt using OpenSSL

openssl_hash_cipher.php

<?php
/**
 * Powerful method to encrypt or decrypt a plain text string
 * initialization vector(IV) has to be the same when encrypting and decrypting
 * 
 * @param string $action: can be 'encrypt' or 'decrypt'
 * @param string $string: string to encrypt or decrypt
 *
 * @return string
 */
function encrypt_decrypt( $action, $string ) {
    $secret_key = 'This is my secret key';
    $cipher = "AES-256-CBC";
    $ivlen = openssl_cipher_iv_length( $cipher );

    if ( $action == 'encrypt' ) {
        $iv = openssl_random_pseudo_bytes( $ivlen );
        $ciphertext_raw = openssl_encrypt( $string, $cipher, $secret_key, $options = OPENSSL_RAW_DATA, $iv );
        $hmac = hash_hmac( 'sha512', $ciphertext_raw, $secret_key, $as_binary = true );
        $ciphertext = base64_encode( $iv.$hmac.$ciphertext_raw );

        return $ciphertext;
    } else if( $action == 'decrypt' ) {
        $c = base64_decode( $string );
        $iv = substr( $c, 0, $ivlen );
        $hmac = substr( $c, $ivlen, $sha2len = 64 );
        $ciphertext_raw = substr( $c, $ivlen+$sha2len );
        $original_plaintext = openssl_decrypt( $ciphertext_raw, $cipher, $secret_key, $options = OPENSSL_RAW_DATA, $iv );
        $calcmac = hash_hmac( 'sha512', $ciphertext_raw, $secret_key, $as_binary = true );

        //PHP 5.6+ timing attack safe comparison
        if ( hash_equals( $hmac, $calcmac ) )
            return $original_plaintext;
        else
            false;
    }
}

$plain_txt = "This is my plain text";
echo "Plain Text = " .$plain_txt. "\n";
$encrypted_txt = encrypt_decrypt( 'encrypt', $plain_txt );
echo "Encrypted Text = " .$encrypted_txt. "\n";
$decrypted_txt = encrypt_decrypt( 'decrypt', $encrypted_txt );
echo "Decrypted Text =" .$decrypted_txt. "\n";
if ( $plain_txt === $decrypted_txt ) echo "SUCCESS";
else echo "FAILED";
echo "\n";
?>