Manipulating parameter values

Hi, I am new to extension programming in php, and I hope you can help me. I want to achieve that I can manipulate the parameters given to my extension functions. I have a function that replaces text. I want that function to return the number of replacements, and to manipulate the string given to the function. To retrieve the parameters i use zend_parse_parameters(ZEND_NUM_ARGS() TSRMLS_CC, ³s², &str, &str_len). But if I manipulate the str variable this change is not passed to the php-caller-script. Do you have any ideas? Thanks, Mathias