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michael dot moussa at gmail dot com ¶
16 years ago
As previous commentators mentioned, when $search contains values that occur earlier in $replace, str_replace will factor those previous replacements into the process rather than operating solely on the original string.  This may produce unexpected output.

Example:

<?php
$search = array('A', 'B', 'C', 'D', 'E');
$replace = array('B', 'C', 'D', 'E', 'F');
$subject = 'ABCDE';

echo str_replace($search, $replace, $subject); // output: 'FFFFFF'
?>

In the above code, the $search and $replace should replace each occurrence in the $subject with the next letter in the alphabet.  The expected output for this sample is 'BCDEF'; however, the actual output is 'FFFFF'.

To more clearly illustrate this, consider the following example:

<?php
$search = array('A', 'B', 'C', 'D', 'E');
$replace = array('B', 'C', 'D', 'E', 'F');
$subject = 'A';

echo str_replace($search, $replace, $subject); // output: 'F'
?>

Since 'A' is the only letter in the $search array that appears in $subject, one would expect the result to be 'B'; however, replacement number $n does *not* operate on $subject, it operates on $subject after the previous $n-1 replacements have been completed.

The following function utilizes array_combine and strtr to produce the expected output, and I believe it is the most efficient way to perform the desired string replacement without prior replacements affecting the final result.

<?php
/**
* When using str_replace(...), values that did not exist in the original string (but were put there by previous
* replacements) will be replaced continuously.  This string replacement function is designed replace the values
* in $search with those in $replace while not factoring in prior replacements.  Note that this function will
* always look for the longest possible match first and then work its way down to individual characters.
*
* The "o" in "stro_replace" represents "original", indicating that the function operates only on the original string.
*
* @param array $search list of strings or characters that need to be replaced
* @param array $replace list of strings or characters that will replace the corresponding values in $search
* @param string $subject the string on which this operation is being performed
*
* @return string $subject with all substrings in the $search array replaced by the values in the $replace array
*/
function stro_replace($search, $replace, $subject)
{
    return strtr( $subject, array_combine($search, $replace) );
}

$search = array('A', 'B', 'C', 'D', 'E');
$replace = array('B', 'C', 'D', 'E', 'F');
$subject = 'ABCDE';

echo stro_replace($search, $replace, $subject); // output: 'BCDEF'
?>

Some other examples:

<?php
$search = array(' ', '&');
$replace = array('&nbsp;', '&amp;');
$subject = 'Hello & goodbye!';

// We want to replace the spaces with &nbsp; and the ampersand with &amp;
echo str_replace($search, $replace, $subject); // output: "Hello&amp;nbsp&amp;&amp;nbspgoodbye!" - wrong!

echo stro_replace($search, $replace, $subject); // output: "Hello&nbsp;&amp;&nbsp;goodbye!" - correct!

/*
    Note:  Run the above code in the CLI or view source on your web browser - the replacement strings for stro_replace are HTML entities which the browser interprets.
*/
?>

<?php
$search = array('ERICA', 'AMERICA');
$replace = array('JON', 'PHP');
$subject = 'MIKE AND ERICA LIKE AMERICA';

// We want to replace the name "ERICA" with "JON" and the word "AMERICA" with "PHP"
echo str_replace($search, $replace, $subject); // output: "MIKE AND JON LIKE AMJON", which is not correct

echo stro_replace($search, $replace, $subject); // output: "MIKE AND JON LIKE PHP", which is correct
?>
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