Add following code.

php.ini
zend.enable_gc = Off
[xdebug]
zend_extension="xdebug.so" ; Path to the Xdebug extension
xdebug.mode=profile ; Enable only profiling (no step debugger)
xdebug.start_with_request=yes ; Do not start profiling automatically on every request
xdebug.output_dir="/Users/arshid/Desktop/test/out" ; Directory to store cachegrind/profile files
xdebug.profiler_output_name="cachegrind.out.%p" ; File name pattern (%p = PID)

<?php //test.php
function a1(){
str_replace("a", '-', "dfdsfaa,aaaadfgfg,fdfdfdfdfdfddfdfdfdfdfdf");
str_replace("ad", '-', "dfdsfaa,aaaadfgfg,fdfdfdfdfdfddfdfdfdfdfdf");
}

function a2(){
str_replace("a", '-', "dfda");
}
a2();
a1();