Highlight the difference between two strings in PHP

Question

What is the easiest way to highlight the difference between two strings in PHP?

I\'m thinking along the lines of the Stack Overflow edit history page, where new text

Answer 1

If you want a robust library, Text_Diff (a PEAR package) looks to be pretty good. It has some pretty cool features.

Answer 2

You were able to use the PHP Horde_Text_Diff package.

However this package is no longer available.

Answer 3

This is the best one I've found.

http://code.stephenmorley.org/php/diff-implementation/

Answer 4

Another solution (for side-by-side comparison as opposed to a unified view): https://github.com/danmysak/side-by-side.

Answer 5

Here is a short function you can use to diff two arrays. It implements the LCS algorithm:

function computeDiff($from, $to)
{
    $diffValues = array();
    $diffMask = array();

    $dm = array();
    $n1 = count($from);
    $n2 = count($to);

    for ($j = -1; $j < $n2; $j++) $dm[-1][$j] = 0;
    for ($i = -1; $i < $n1; $i++) $dm[$i][-1] = 0;
    for ($i = 0; $i < $n1; $i++)
    {
        for ($j = 0; $j < $n2; $j++)
        {
            if ($from[$i] == $to[$j])
            {
                $ad = $dm[$i - 1][$j - 1];
                $dm[$i][$j] = $ad + 1;
            }
            else
            {
                $a1 = $dm[$i - 1][$j];
                $a2 = $dm[$i][$j - 1];
                $dm[$i][$j] = max($a1, $a2);
            }
        }
    }

    $i = $n1 - 1;
    $j = $n2 - 1;
    while (($i > -1) || ($j > -1))
    {
        if ($j > -1)
        {
            if ($dm[$i][$j - 1] == $dm[$i][$j])
            {
                $diffValues[] = $to[$j];
                $diffMask[] = 1;
                $j--;  
                continue;              
            }
        }
        if ($i > -1)
        {
            if ($dm[$i - 1][$j] == $dm[$i][$j])
            {
                $diffValues[] = $from[$i];
                $diffMask[] = -1;
                $i--;
                continue;              
            }
        }
        {
            $diffValues[] = $from[$i];
            $diffMask[] = 0;
            $i--;
            $j--;
        }
    }    

    $diffValues = array_reverse($diffValues);
    $diffMask = array_reverse($diffMask);

    return array('values' => $diffValues, 'mask' => $diffMask);
}

It generates two arrays:

• values array: a list of elements as they appear in the diff.
• mask array: contains numbers. 0: unchanged, -1: removed, 1: added.

If you populate an array with characters, it can be used to compute inline difference. Now just a single step to highlight the differences:

function diffline($line1, $line2)
{
    $diff = computeDiff(str_split($line1), str_split($line2));
    $diffval = $diff['values'];
    $diffmask = $diff['mask'];

    $n = count($diffval);
    $pmc = 0;
    $result = '';
    for ($i = 0; $i < $n; $i++)
    {
        $mc = $diffmask[$i];
        if ($mc != $pmc)
        {
            switch ($pmc)
            {
                case -1: $result .= '</del>'; break;
                case 1: $result .= '</ins>'; break;
            }
            switch ($mc)
            {
                case -1: $result .= '<del>'; break;
                case 1: $result .= '<ins>'; break;
            }
        }
        $result .= $diffval[$i];

        $pmc = $mc;
    }
    switch ($pmc)
    {
        case -1: $result .= '</del>'; break;
        case 1: $result .= '</ins>'; break;
    }

    return $result;
}

Eg.:

echo diffline('StackOverflow', 'ServerFault')

Will output:

S<del>tackO</del><ins>er</ins>ver<del>f</del><ins>Fau</ins>l<del>ow</del><ins>t</ins> 

StackOerverfFaulowt

Additional notes:

• The diff matrix requires (m+1)*(n+1) elements. So you can run into out of memory errors if you try to diff long sequences. In this case diff larger chunks (eg. lines) first, then diff their contents in a second pass.
• The algorithm can be improved if you trim the matching elements from the beginning and the end, then run the algorithm on the differing middle only. A latter (more bloated) version contains these modifications too.

Answer 6

There is also a PECL extension for xdiff:

• http://php.net/manual/en/book.xdiff.php
• http://pecl.php.net/package/xdiff

In particular:

• xdiff_string_diff — Make unified diff of two strings

Example from PHP Manual:

<?php
$old_article = file_get_contents('./old_article.txt');
$new_article = $_POST['article'];

$diff = xdiff_string_diff($old_article, $new_article, 1);
if (is_string($diff)) {
    echo "Differences between two articles:\n";
    echo $diff;
}