The md5() function calculates the MD5 hash of a string which is a one pass way of encrypting a string. Here it is important to know the meaning of one pass: "One pass means that from the result you can not get the original string back using the current computing power".
Let see what RFC-1321 says about the MD5 Algorithm - "The MD5 message-digest algorithm takes as input a message of arbitrary length and produces as output a 128-bit "fingerprint" or "message digest" of the input. The MD5 algorithm is intended for digital signature applications, where a large file must be "compressed" in a secure manner before being encrypted with a private (secret) key under a public-key crypto system such as RSA."
SYNTAX of MD5
string: any valid string and must
raw: optional (can have true or false where as false is default behavior)
TRUE - Raw 16 character binary format
FALSE - Default. 32 character hex number
$str = "QueryHome";