<?php
// just do this...
$matches = array();
preg_match_all('/\s(:\w+)/', 'jhkjh :fgfgfgfg :fgfg : : ffgg', $matches);
print_r($matches);
// we could wrap it in a function that eliminates the pass by reference, and/or use Flux (https://github.com/selvinortiz/flux) and extend it
$matches = array();
preg_match_all('/\s(:\w+)/', 'jhkjh :fgfgfgfg :fgfg : : ffgg', $matches, PREG_SET_ORDER);
print_r($matches);