Given the amount of functions in early PHP (we are talking PHP/FI 2 max here, even userlevel code hardly had functions back then) even not hashing them at all,

Given the amount of functions in early PHP (we are talking PHP/FI 2 max here, even userlevel code hardly had functions back then) even not hashing them at all, but doing a linear search over the function table would have been acceptable. Hashing by length most likely lead to few buckets with more than 3 functions.