str_replace for distinct word

Question

suppose I have $str=\"nikparsa neginnikparsa somenikparsa\" when I use

$str= str_replace(\'nikparsa\', \'nik parsa\', $str);
echo $str;

Answer 1

Use preg_replace:

$str = preg_replace('/\bnikparsa\b/i', 'nik parsa', $str);