PHP Developer’s Dictionary IT-SC book 95 Table 4.5. Truth Table for the and Operator variable1 variable2 Result False False False False True False True False False True True True or Syntax variable1 or variable2 Description The or operator evaluates whether either variable1 or variable2 is true. Table 4.6 contains the truth table for the possible values. In understanding the truth table, you can see the only time the equation returns false is when both variable1 and variable2 are false. Note The only difference between this operator and the || operator is order precedence. Table 4.6. Truth Table for the or Operator variable1 variable2 Result False False False False True True True False True True True True xor Syntax variable1 xor variable2 PHP Developer’s Dictionary IT-SC book 96 Description The xor, or exclusive or operator, evaluates whether either variable1 and variable2 , but not both, is true. Table 4.7 contains the truth table for the possible values. In understanding the truth table, you can see when variable1 and variable2 are the same, false is returned. Table 4.7. Truth Table for the xor Operator variable1 variable2 Result False False False False True True True False True True True False ! (not) Syntax !variable Description The ! , or not operator, returns the opposite of variable . For instance, if variable is true, applying the ! operator will return false. && (and) Syntax variable1 && variable2 Description The && operator evaluates whether variable1 and variable2 are true, or equal. Table 4.8 contains the truth table for the possible values. In understanding the truth table, you can see the only time the equation returns true is when both variable1 and variable2 are true. Note The only difference between this operator and the and operator is order precedence. PHP Developer’s Dictionary IT-SC book 97 Table 4.8. Truth Table for && Operator variable1 variable2 Result False False False False True False True False False True True True || (or) Syntax variable1 || variable2 Description The || operator evaluates whether either variable1 or variable2 is true. Table 4.9 contains the truth table for the possible values. In understanding the truth table, you can see the only time the equation returns false is when both variable1 and variable2 are false. Note The only difference between this operator and the or operator is order precedence. Table 4.9. Truth Table for the || Operator variable1 variable2 Result False False False False True True True False True True True True String The string operators are used to perform manipulation on the string data types. Although some of the previous operators discussed in this section can be performed on strings as well, these are limited to strings only. . (Concatenation) PHP Developer’s Dictionary IT-SC book 98 Syntax string1 . string2 Description The . , or concatenation operator, will "add" string2 to string1 when applied. For instance, if string1 ="P" and string2 ="HP" , string1 . string2 would equal "PHP" . $string1 = "hel"; $string2 = "p"; $string3 = "lo"; $result1 = $sting1 . $string2; // $result1 has "help" $result2 = $sting1 . $string3; // $result2 has "hello" .= (Concatenating Assignment) Syntax string1 .= string2 Description The . , or concatenating assignment operator, will "add" string2 to string1 when applied and then store the new string in string1 . For instance, if string1 ="P" and string2 ="HP" , then string1 .= string2 would equal "PHP" and that would be the new value of string1 . Predefined Variables Predefined variables in PHP refer to language elements that are consistent across all applications run in that environment. This is very much like the environment variables you see within the UNIX and Windows operating systems. (Type env at a command line to see a list of your operating system variables.) Apache The Apache predefined variables reflect the environment settings of your Apache Web server when running PHP. There are also variables that reflect information about a request of a given user-agent, or browser. This enables you to grab the requested URL, query string, or another element of the HTTP request. PHP Developer’s Dictionary IT-SC book 99 DOCUMENT_ROOT Syntax string DOCUMENT_ROOT Description The DOCUMENT_ROOT variable contains the document root, as defined in the PHP configuration file, under which the current script is being parsed. GATEWAY_INTERFACE Syntax string GATEWAY_INTERFACE Description The GATEWAY_INTERFACE variable contains the version of the Common Gateway Interface (CGI) specification that the server is using. For instance, CGI/1.1 is a valid GATEWAY_INTERFACE. HTTP_ACCEPT Syntax string HTTP_ACCEPT Description The HTTP_ACCEPT variable contains the contents of the HTTP Accept: header if the user-agent sent it to the server. HTTP_ACCEPT_CHARSET Syntax string HTTP_ACCEPT_CHARSET . then string1 .= string2 would equal " ;PHP& quot; and that would be the new value of string1 . Predefined Variables Predefined variables in PHP refer to language elements that are consistent. request. PHP Developer’s Dictionary IT-SC book 99 DOCUMENT_ROOT Syntax string DOCUMENT_ROOT Description The DOCUMENT_ROOT variable contains the document root, as defined in the PHP configuration. Note The only difference between this operator and the and operator is order precedence. PHP Developer’s Dictionary IT-SC book 97 Table 4.8. Truth Table for && Operator variable1