This function compress the given string using the ZLIB data format.
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.
The data to compress.
The level of compression. Can be given as 0 for no compression up to 9 for maximum compression.