If you want a very simple and short solution to insert string into another string use:
substr_replace ( $string , $replacement , $start , 0 ). The key here is zero, and because we set $length to zero this function will have the effect of inserting the replacement into the string.
<?php $string = 'I am happy today.'; $replacement = 'very '; echo substr_replace($string, $replacement, 4, 0); // I am very happy today.