@Al_Grigor's challenge

challenge.php

<?php

function process( $input)
{
        $result = '[';

        $occurences = array();
        $occur_offset = -1;

        $input_sz = strlen($input);
        for( $i=0; $i < $input_sz; ++$i)
        {
                if( $occur_offset < 0 || $occurences[$occur_offset]['char'] != $input[$i] )
                {
                        ++$occur_offset;
                        $occurences[$occur_offset]['char'] = $input[$i];
                        $occurences[$occur_offset]['count'] = 1;
                        continue;
                }

                // Increment the occurence counter
                ++$occurences[$occur_offset]['count'];
        }

        $is_first_pair = true;
        for( $i=0; $i<count($occurences); ++$i)
        {
                if( !$is_first_pair )
                        $result .= ',';
                $result .= '("' . $occurences[$i]['char'] . '", ' . $occurences[$i]['count'] . ')';
                $is_first_pair = false;
        }

        $result .= ']';
        return $result;
}


echo( process("aaaabbbcca") );