base64_decode

(PHP 4, PHP 5, PHP 7)

base64_decode — 对使用 MIME base64 编码的数据进行解码

说明

base64_decode ( string $data [, bool $strict = false ] ) : string

对 base64 编码的 data 进行解码。

参数

data

编码过的数据。

strict

当设置 strict 为 TRUE 时，一旦输入的数据超出了 base64 字母表，将返回 FALSE。 否则会静默丢弃无效的字符。

返回值

返回原始数据， 或者在失败时返回 FALSE。返回的数据可能是二进制的。

更新日志

版本 说明
5.2.0 增加了 strict 。

范例

Example #1 base64_decode() 示例

<?php
$str = 'VGhpcyBpcyBhbiBlbmNvZGVkIHN0cmluZw==';
echo base64_decode($str);
?>

以上例程会输出：

This is an encoded string

参见

User Contributed Notes

user at sfdsfd dot com 07-Jul-2019 08:03
function is_base64($str){
        if($str === base64_encode(base64_decode($str))){
            return true;
        }
        return false;
    }

---------------------------------------------------------------------------
---------------------------------------------------------------------------
---------------------------------------------------------------------------

    $str = 'VGhpcyBpcyBiYXNlNjQgZW5jb2RlIHN0cmluZw==';

    if(is_base64($str)){
        print base64_decode($str);
    }
markandrewslade at gmail dot com 08-Feb-2016 04:34
The docs don't make this explicitly clear, but if you omit `$strict` or set it to `false` then invalid characters in the encoded input will be silently ignored.

<?php

echo base64_decode('VGhpcyBpcyBhbiBlbmNvZGVkIHN0cmluZw=='); // "This is an encoded string"
echo base64_decode('$VGhpcyBpcyBhbiBlbmNvZGVkIHN0cmluZw=='); // "This is an encoded string"
echo base64_decode('$VGhpcyBpcyBhbiBlbmNvZGVkIHN0cmluZw==', true); // false

?>
walf 02-Nov-2015 06:58
Base64 for URL parameters/filenames, that adhere to RFC 4648.
Defaults to dropping the padding on encode since it's not required for decoding, and keeps the URL free of % encodings.

<?php
function base64url_encode($data, $pad = null) {
    $data = str_replace(array('+', '/'), array('-', '_'), base64_encode($data));
    if (!$pad) {
        $data = rtrim($data, '=');
    }
    return $data;
}
function base64url_decode($data) {
    return base64_decode(str_replace(array('-', '_'), array('+', '/'), $data));
}
nicolem2005 at gmail dot com 23-Aug-2011 05:17
base64_decode seems to fail when decoding big files/strings. I had an issue decoding a 7MB image file. Here is a solution that worked for me:

$decodedstring=base64_decode(chunk_split($encodedstring));
winkelnkemper at googlemail dot com 27-Jan-2011 04:25
If you want to save data that is derived from a Javascript canvas.toDataURL() function, you have to convert blanks into plusses. If you do not do that, the decoded data is corrupted:

<?php
  $encodedData = str_replace(' ','+',$encodedData);
  $decocedData = base64_decode($encodedData);
?>
martinstaemmler at gmx dot net 17-Aug-2009 09:05
I had some trouble trying to let base64_decode decode base64-strings longer than ~5k chars.

The base64-decoding function is a homomorphism between modulo 4 and modulo 3-length segmented strings. That motivates a divide and conquer approach: Split the encoded string into substrings counting modulo 4 chars, then decode each substring and concatenate all of them.

Then instead of

<?php $decoded = base64_decode($encoded); ?>

for big $encoded strings, it's saver to use

<?php
$decoded = "";
for ($i=0; $i < ceil(strlen($encoded)/256); $i++)
   $decoded = $decoded . base64_decode(substr($encoded,$i*256,256));
?>

where 256 can be replaced by a sufficiently small modulo 4 natural.
dimagolov at yahoo dot com 10-Jul-2009 11:44
Here is function to decode Base 62 (see http://en.wikipedia.org/wiki/Base_62) string to number. It is used by MTA in message id, e.g. by Exim
<?php
function base62_decode($str) {
    $ret= 0;
    for ($i= 0, $l= strlen($str); $i < $l; $i++) {
        $val= ord($str[$i]);
        if (ctype_digit($str[$i]))
            $val-= ord('0');
        else if (ctype_upper($str[$i]))
            $val-= ord('A') - 10;
        else if (ctype_lower($str[$i]))
            $val-= ord('a') - 36;
        else
            $val= 0;
        $ret= $ret * 62 + $val;
    }
    return $ret;
}
?>
mcalwell 09-Apr-2009 09:20
I had a problem testing whether an imap message body was base64 encoded on a pre 5.2.* server.  I had been using this function on a post 5.2 server.

I found that the function imap_base64() returns FALSE on failing to decode a string, and that I could use that to check instead.

<?php
if(imap_base64($body)) $body = imap_base64($body);
?>
alvaro at demogracia dot com 05-Dec-2008 01:11
You can do partial decoding (e.g. from buffered input streams) if you choose a chunk length that is multiple of 4:

<?php

$encoded = base64_encode('The quick brown fox jumps over the lazy dog');
for($i=0, $len=strlen($encoded); $i<$len; $i+=4){
    echo base64_decode( substr($encoded, $i, 4) );
}

?>

4 encoded chars represent 3 original chars. The "=" character is used as padding.
twm at twmacinta dot com 09-Jul-2008 08:38
To follow up on Starson's post, PHP was changed to no longer treat a space as if it were a plus sign in CVS revision 1.43.2.1, which corresponds to PHP 5.1.0.  You can see what happened with a diff to branch point 1.43 at:

http://cvs.php.net/viewvc.cgi/php-src/ext/standard/base64.c

The CVS log indicates that this change was made to fix bug #34214 (base64_decode() does not properly ignore whitespace).

It would seem from the comment preceding the code which was removed that the treatment of the space as if it were the plus sign was actually intentional at one time:

    When Base64 gets POSTed, all pluses are interpreted as spaces.
    This line changes them back.  It's not exactly the Base64 spec,
    but it is completely compatible with it (the spec says that spaces
    are invalid).  This will also save many people considerable
    headache.

    if (ch == ' ') ch = '+';

However, RFC 3548 states that characters not in the Base64 alphabet should either be ignored or cause the implementation to reject the encoding and RFC 2045 says they should be ignored.  So the original code was unfortunately not fully compatible with the spec or other implementations.  It may have also masked problems with code not properly escaping POST variables.
debug 27-Feb-2008 12:39
@morgangalpin att gmail dotty com

A better implementation would be the following regular expression:

^[a-zA-Z0-9/+]*={0,2}$

Which will also detect the usage of = or == at the end of the string (and only end).

If this regex isn't following proper RFC guidelines, please comment on it.

A function geared specifically toward this:

<?php

function is_base64_encoded()
    {
        if (preg_match('%^[a-zA-Z0-9/+]*={0,2}$%', $data)) {
            return TRUE;
        } else {
            return FALSE;
        }
    };

is_base64_encoded("iash21iawhdj98UH3"); // true
is_base64_encoded("#iu3498r"); // false
is_base64_encoded("asiudfh9w=8uihf"); // false
is_base64_encoded("a398UIhnj43f/1!+sadfh3w84hduihhjw=="); // true

?>
zmorris at zsculpt dot com 10-Nov-2007 02:22
Here is a drop-in replacement for base64_decode(), based on a faster version of morgangalpin's code:

<?php
// workaround for bug in php 4.3.11 through 4.4.7, 5.1.2 through 5.2.4 and perhaps others (http://bugs.php.net/bug.php?id=37244)
function    base64_decode_fix( $data, $strict = false )
{
    if( $strict )
        if( preg_match( '![^a-zA-Z0-9/+=]!', $data ) )
            return( false );
   
    return( base64_decode( $data ) );
}
?>
Tom 06-Dec-2006 10:23
This function supports "base64url" as described in Section 5 of RFC 4648, "Base 64 Encoding with URL and Filename Safe Alphabet"

    <?php
    function base64url_decode($base64url)
    {
        $base64 = strtr($base64url, '-_', '+/');
        $plainText = base64_decode($base64);
        return ($plainText);
    }
    ?>
Starson 19-Sep-2006 06:23
To expand on Jes' post:

The change took place between 5.0.5 and 5.1.0.  Exactly where I don't know or care.

In short php <= 5.0.5's base64_decode( $string ) will assume that a space is meant to be a + sign where php >= 5.1.0's base64_decode( $string ) will no longer make that assumption.  I did not see this noted in the change log.

Please note that, as of this writing, mb_convert_encoding( $string, "UTF-8", "BASE64" ) still behaves as base64_decode( $string ) did in php <= 5.0.5 regardless of the version of php you are running.
tobias at silverxnet dot de 23-Dec-2003 12:16
I was wondering how to decode attached images within mails. Basically they are mostly JPEG files, so it was obviously to write a function that decodes JPEG images.
I guess the plainest way to do so was the following:

<?php
function base64_to_jpeg( $inputfile, $outputfile ) {
  /* read data (binary) */
  $ifp = fopen( $inputfile, "rb" );
  $imageData = fread( $ifp, filesize( $inputfile ) );
  fclose( $ifp );
  /* encode & write data (binary) */
  $ifp = fopen( $outputfile, "wb" );
  fwrite( $ifp, base64_decode( $imageData ) );
  fclose( $ifp );
  /* return output filename */
  return( $outputfile );
}
?>

This function decodes the given inputfile (a filename!) and saves it to the given outputfile (a filename as well) and then returns the output filename for further usage (e.g. redirect, imagejpeg() and so on).
I thought that might be helpful.
Klaus Fehrenbacher 17-Apr-2003 04:05
this script can correct the bug

<?php
$enc = chunk_split(preg_replace('!\015\012|\015|\012!','',$enc));
$enc = base64_decode($enc);
?>
nsayer at kfu dot com 21-Mar-2002 01:15
I used to do uudecode as a C module, but I've discovered a really fast way to do it in PHP. Here it is:

<?php
function uudecode($encode) {
  $b64chars="ABCDEFGHIJKLMNOPQRSTUVWXYZ\
abcdefghijklmnopqrstuvwxyz0123456789+/";

  $encode = preg_replace("/^./m","",$encode);
  $encode = preg_replace("/\n/m","",$encode);
  for($i=0; $i<strlen($encode); $i++) {
    if ($encode[$i] == '`')
      $encode[$i] = ' ';
    $encode[$i] = $b64chars[ord($encode[$i])-32];
  }

  while(strlen($encode) % 4)
    $encode .= "=";

  return base64_decode($encode);
}
?>

This is the PHP equivalent to perl's unpack("u",___). That is, you need to strip the 'begin' and 'end' lines from the typical uuencoded file.