Replace words found in string with highlighted word keeping their case as found

To highlight a single word case-insensitively

Use preg_replace() with the following regex:

/\b($p)\b/i

Explanation:

• / - starting delimiter
• \b - match a word boundary
• ( - start of first capturing group
• $p - the escaped search string
• ) - end of first capturing group
• \b - match a word boundary
• / - ending delimiter
• i - pattern modifier that makes the search case-insensitive

The replacement pattern can be <span style="background:#ccc;">$1</span>, where $1 is a backreference — it would contain what was matched by the first capturing group (which, in this case, is the actual word that was searched for)

Code:

$p = preg_quote($word, '/');  // The pattern to match

$string = preg_replace(
    "/\b($p)\b/i",
    '<span style="background:#ccc;">$1</span>', 
    $string
);

To highlight an array of words case-insensitively

$words = array('five', 'colors', /* ... */);
$p = implode('|', array_map('preg_quote', $words));

$string = preg_replace(
    "/\b($p)\b/i", 
    '<span style="background:#ccc;">$1</span>', 
    $string
);

var_dump($string);