- str_replace: documentation ( source)
- preg_replace: documentation ( source)
<?php
$t = '/"(securityPin)[\]*":[\]*"(.)[^"\]+[\]*"/';
echo preg_replace($t, '"\${1}\":\"${2}***\"', str_replace('":""', '":"*"', '3\",\"securityPin\":\"35515\",\"salutation'));