I know that hashing functions such as SHA1 and MD5 are one-way encryption systems.
But is there a hashing method which is 'dehashable'?
Like, it produces an x-character string, which can then be 'dehashed' into the original string.
Is there such a hashing method? It will be appreciated if it was PHP compatible.
UPDATE: What I mean by a hashing function is an encryption method which produces an x-character string, which can be decrypted. Sorry for the confusion.
hashing functions such as SHA1 and MD5 are one-way encryption systems.
Not quite - they are as you say hashing functions. They are often used together with encryption systems, e.g. for password hashing algorithms, but they are not encryption systems or encryption algorithms.
But is there a hashing method which is 'dehashable'?
No, it would not be a hash function then, since a hash function maps a larger data set to a smaller data set. This has the side effect that you can get the same hash value out of different input data, which makes calculating the original data from the hash key impossible. What it does allow is, for instance, to check if the original data has been modified - you apply the same hash function to the original data again and compare the calculated hash keys. If they are different, the original data was modified - if they are the same, the original data is (at least very very likely) unmodified.
What you are looking for is probably either a compression/decompression algorithm or an encryption/decryption algorithm.
Hashing is not (one-way) encryption, as a hash value can never be decrypted to the original value; this is by design.
Also, hash functions are designed to make it very hard to come up with a data set the will match a given hash value (cf. collision)
As Andreas suggests, you are looking for compression or crypto functions.