this intended behaviour that is configurable
and defaults to 'on'

see http://www.php.net/manual/en/configuration.php
and check your magic_quotes_* settings in php.ini 
(verify using phpinfo() output)



magic_quotes_gpc  boolean

    Sets the magic_quotes state for GPC (Get/Post/Cookie) operations. When magic_quotes are on, all ' (single-quote), " (double quote), \ (backslash) and NUL's are escaped with a backslash automatically. If magic_quotes_sybase is also on, a single-quote is escaped with a single-quote instead of a backslash.


magic_quotes_runtime boolean

    If magic_quotes_runtime is enabled, most functions that return data from any sort of external source including databases and text files will have quotes escaped with a backslash. If magic_quotes_sybase is also on, a single-quote is escaped with a single-quote instead of a backslash.


magic_quotes_sybase boolean

    If magic_quotes_sybase is also on, a single-quote is escaped with a single-quote instead of a backslash if magic_quotes_gpc or magic_quotes_runtime is enabled.