str_ireplace()

The str_ireplace() function in PHP replaces all occurrences of a search string with a replacement string, ignoring case. It is the case-insensitive twin of str_replace(): everything works the same way, except the search is not sensitive to upper- and lower-case letters. This makes str_ireplace() the right choice when the casing of the target text is unknown or inconsistent — for example, when cleaning up user input or normalizing tags that may appear as PHP, php, or Php.

This article covers the syntax of str_ireplace(), how each parameter behaves, how to work with arrays, how to count replacements, and the gotchas to watch out for.

Syntax

str_ireplace(
    array|string $search,
    array|string $replace,
    string|array $subject,
    int &$count = null
): string|array

The function takes three required parameters and one optional one:

• $search — the value (or array of values) to look for.
• $replace — the value (or array of values) to replace it with.
• $subject — the string (or array of strings) to search inside.
• $count (optional) — a variable passed by reference that is filled with the total number of replacements performed.

It returns the modified $subject. If $subject is an array, an array is returned with the replacement applied to every element.

Basic example

Even though we search for "FOX" in uppercase, str_ireplace() ignores case and matches the lowercase "fox" in the subject. The output is:

The quick brown cat jumps over the lazy dog

With the standard, case-sensitive str_replace(), the same call would find no match and the string would be returned unchanged.

Using arrays for search and replace

When $search and $replace are both arrays, each search value is paired with the replacement at the same index. This lets you perform several substitutions in one call:

<?php
$search  = ["red", "GREEN", "Blue"];
$replace = ["crimson", "emerald", "navy"];
$subject = "Red, green and blue are colors.";

echo str_ireplace($search, $replace, $subject);
// crimson, emerald and navy are colors.
?>

Notice that the casing of the entries in $search does not matter — "GREEN" still matches "green". If $replace has fewer elements than $search, the missing replacements are treated as empty strings (the matches are removed).

When $search is an array but $replace is a single string, that one string replaces every search value:

<?php
echo str_ireplace(["cats", "dogs"], "pets", "I have CATS and Dogs.");
// I have pets and pets.
?>

Counting replacements with $count

Pass a fourth argument by reference to learn how many replacements were made:

<?php
$text   = "PHP is great. php is fun. PhP rocks.";
$result = str_ireplace("php", "PHP", $text, $count);

echo $result . "\n";       // PHP is great. PHP is fun. PHP rocks.
echo "Replacements: $count"; // Replacements: 3
?>

The three variants PHP, php, and PhP are all matched, so $count is 3. This is handy for validation — for example, checking whether a forbidden word appeared at all.

Gotchas to keep in mind

• The replacement is not searched again. str_ireplace() processes search terms left to right and never re-scans text it has already inserted, so it cannot loop infinitely. But order still matters when one search term is a substring of another.
• It operates on bytes, not multibyte characters. For UTF-8 text where case-insensitivity must respect accented or non-Latin letters, str_ireplace() may not behave as expected. Use PHP's mb_* string functions or preg_replace() with the i modifier instead.
• Use str_replace() when case matters. If you need exact, case-sensitive matching, reach for str_replace() — it is slightly faster because it skips the case-folding step.

Summary

str_ireplace() performs case-insensitive search-and-replace on strings. Use it for simple, literal substitutions where casing should be ignored; use str_replace() when case matters, and preg_replace() when you need pattern-based or multibyte-aware matching.

Practice