Question

Example:

string1="ah ah I love you ah ah ah ah";

After replacement:

string1="ah ah I love you ah thank you ah ah";

The above means ‘ah’ at fourth position should be replaced by ‘thank you’

I don’t know how to code task above by PHP. Can you help me?

Answer 1

$string = "ah ah I love you ah ah ah ah";
echo preg_replace_callback('/ah/', function($m) {
    static $count = 0;
    if(++$count == 4) return 'thank you';
    else return $m[0];
}, $string);

How it works: Each time ah matches the callback function is invoked. The static $count variable is increased and when it’s the fourth match it returns the replacement string, otherwise the originally matched string.