What is the source of your two strings? Are they from the results from a database query?
It may be a co-incidence but both search strings are six characters long and both occur in the source string starting at the second character ( 0 is the first character).
If on the off-chance the string you are searching always occurs in the same place then you could use the following:
echo substr( $s1, 2, 6); # ccddkk
echo substr( $s2, 2, 6); # 334466