How to get all combinations of a string with fixed character position?

I need to find a way to produce all combinations of a string with a p开发者_高级运维articular character to always display in PHP.

For example, given a string 'ABCD', and I want to get all combinations of the string with the character 'B' present, I want to get:

array(' B ', ' BC ', ' BCD', ' B D', 'AB ', 'ABC ', 'AB D', 'ABCD')

The missing characters are replaced with spaces. Anyone have any ideas?

It's easy if you think of each letter in the string being either "on" or "off" - like a bit in a binary number. In fact, you can represent it as such.

So think of your string as a four bit number, which can be anything from 0b0000 = 0 = "" to 0b1111 = 15 = "ABCD". Then you can just run through all the numbers from 0 to 15, and find the respective "permutation" by seeing what bits are set.

For example, "permutation" 6: 0b0110 -> " BC "

Hope that helps!

PS: If this is homework, you should tag it as such - it's a bit of a faux pas around here not to.

PPS: Your "permutations" are actually "combinations." Linked to Wikipedia just in case you're curious.