This function returns a compressed version of the input data using the ZLIB data format, or false if an error is encountered. The optional parameter level can be given as 0 for no compression up to 9 for maximum compression.
For details on the ZLIB compression algorithm see the document "ZLIB Compressed Data Format Specification version 3.3" (RFC 1950).
Note: This is not the same as gzip compression, which includes some header data. See gzencode() for gzip compression.