photoshop learning tutorial PHP learning PHP variables

PHP variables
　 PHP3 supports the following types of variables:
(1), internal variables
　 Mainly include integers, float-point numbers, strings, arrays, and objects.
1 Initialize variables
To initialize a variable in PHP, you just need to simply assign a value to it. For most types, this is the most straightforward. For arrays and objects, other methods are available.
2 Initializing an Array
An array can be assigned using one of two methods: using a series of consecutive values, or constructing it using the array() function (see the Array functions section).
To add consecutive values ​​to an array, you only need to assign the value to the array variable without a subscript. This value will be added to the array as the last element of the array.
Example: 　　$names[] = "Jill"; // $names[0] = "Jill" 　　$names[] = "Jack"; // $names[1] = "Jack" Similar to c and perl,
Array subscripts also start from 0.
3 Initializing the object
To initialize an object, you need to use the new statement to create a variable of this type.
class foo { 　　　　
function do_foo() { 　　　　
echo "Doing foo."; } 　　　　 $bar = new foo;
The scope of a variable is its validity scope. For most PHP variables there is only one scope. Use local variable scope in user-defined functions.
Variables used within functions are set to local variables by default. For example: $a=1; /* global scope */
Function Test() { echo $a; /* reference to local scope variable */ }
Test(); This program will not output anything because echo The statement wants to output the local variable $a, but $a within the function has never been assigned a value.
You may notice that this is a little different from the C language. In C, global variables can be directly referenced within the function unless it is overwritten by a local variable.
And this makes it possible for people to modify the value of global variables without noticing. In PHP, global variables must be used explicitly within a function.
For example: 　　　$a=1; 　　　$b=2; 　　　Function Sum() { 　　　　global $a,$b; 　　　
　$b = $a + $b; echo $b; The above program will output "3 ".
By declaring $a and $b as global variables inside the function, all required variables refer to the global world. There is no limit to the number of global variables that a function can manipulate.
Another noteworthy aspect of scope is static variables.
A static variable exists in a local function, but its value is not lost when the program leaves the function.
Consider the following example: Function Test() { $a=0; echo $a; $a++; }
This function is useless because every time it is called, it first sets $a to 0 and then prints "0 ". The self-increment of $a++ has no effect because the variable
$a is released after the function call ends. To make the counting program count effectively without losing the current counting result, $a must be declared as a static variable:
　　　Function Test() { 　　　static $a=0; 　　　echo $a; 　　　$a++;
　　　} Now, every Every time the Test() function is called, it will print the value of $a and increase its value. Static variables are essential when using recursive functions.
A recursive function is a function that calls itself. Be very careful when writing recursive functions, because the number of loops is uncertain. You must ensure that there are sufficient conditions to end the recursive process. Here is a simple recursive function to count to 10:
　　　Function Test() { 　　　　static $count=0; 　　　$count++; 　　　
echo $count; 　　　if($count （二) Dynamic variables Sometimes it is more convenient to use variable variable names. That is, a variable name that can be dynamically assigned and used.
The assignment statement of an ordinary variable is such as: 　　$a = "hello"; 　　A dynamic variable refers to the value of the variable as the name of a new variable.
In the above example, hello can be used as a variable name by double $.
Example: 　　$$a = "world"; At this point, two variables are defined and stored in the PHP symbol tree: the content of $a is "hello", and the content of $hello is "world".
Therefore, the display result of the statement: echo "$a ${$a}"; is exactly the same as: echo "$a $hello"; (3) PHP external variables 1. HTML form (GET and POST)
When a form When submitting to PHP3 script, PHP will automatically get the variables in the form.For example: 　　　
　　　Name:
When "submit" is pressed, PHP3 will automatically generate the variable: $name, which contains all the content entered by the user. 2. IMAGE SUBMIT variable name
When submitting a form, you can replace the standard submit button with an image through the following markup: When the user clicks on the image,
Two additional variables sub_x and sub_y will be sent to the server along with the form superior. It contains the coordinates of where the user clicked on the graph.
Experienced people may notice that the name actually sent by the browser contains a period instead of an underscore, but PHP automatically converts the period into an underscore.
３、HTTP Cookies
PHP supports HTTP cookies. Cookies store data in the client's browser to keep in touch with the user or authenticate the user's identity.
You can use the setcookie() function to set cookies. Cookies are part of the HTTP request header, so the SetCookie() function must be called before any output data is returned to the user's browser. It is similar to the limitation of the Header() function. Any cookies returned from the client will be automatically converted into standard PHP variables, just like data for GET and POST methods.
If you want to set multiple values ​​​​in a cookie, add [] to the name of the cookie,
For example: SetCookie("MyCookie[]","Testing", time()+3600);
Note: New The cookie will overwrite an existing cookie with the same name in your browser unless they have a different path or domain.
４. Environment variables
PHP automatically converts environment variables into ordinary variables.
echo $HOME; /* Shows the HOME environment variable, if set. */
Although information from GET, POST and Cookie structures will also be automatically converted into PHP variables, it is best to explicitly read them from the environment variables to ensure you get the correct value.
Use the getenv() function for this. Variables can also be set via the putenv() function.
Variable type conversion
PHP does not require (and does not support) explicit type declarations when defining variables; the type of a variable depends on the type of its value.
In other words, if you assign a string value to the variable var, var becomes a string variable. If you assign an integer value to var, it becomes an integer variable.
An example of PHP automatic type conversion is the addition operator '+'. If any operand is of double type, all operands are calculated as double type, and the result is also of double type. Otherwise, all operands are calculated as integer types, and the result is also of integer type. Note: The type of the operand itself does not change;
Type conversion is only done during calculation $foo = "0"; // $foo is a string (ASCII 48) 　　$foo++; // $foo is the string "1" (ASCII 49)
　　$foo += 1; // $foo is now an integer (2) 　　$foo = $foo + 1.3; // $foo is now a double (3.3) 　　
$foo = 5 + "10 Little Piggies"; // $foo is a double (15) 　　$foo = 5 + "10 Small Pigs"; // $foo is an integer (15)
To change the type of the variable, you can also use the settype() function.
1. Forced type conversion
Forced type conversion in PHP is the same as in C: write the desired type name in parentheses before the variable that needs to be typed.
$foo = 10; // $foo is an integer 　　$bar = (double) $foo; // $bar is a double 　　
The allowed casts are: 　　 (int), (integer) - cast to integer 　　 (real) , (double), (float) - cast to double 　　
(string) - cast to string 　　 (array) - cast to array 　　 (object) - cast to object
Note: The brackets can contain tabs or spaces, the following function will be Calculation: $foo = (int) $bar; $foo = (int) $bar;
2. String conversion
When a string is calculated as a numerical type, the value and type of the result are determined as follows.
If the string contains any '.', 'e', ​​and 'E' characters, it is calculated as a double type. Otherwise, it is calculated as an integer type. ​
This value is calculated from the beginning of the string. If the string is a legal number, this value is used, otherwise the value is 0.
A legal number is a sign bit (optional), followed by one or more digits (it can also contain a decimal point), followed by an optional exponent.
The exponent is an 'e' or 'E' followed by one or more digits. $foo = 1 + "10.5"; // $foo is a double (11.5) 
$foo = 1 + "-1.3e3"; // $foo is a double (-1299)  $foo = 1 + "bob- 1.3e3"; // $foo is a double (1) 
$foo = 1 + "bob3"; // $foo is an integer (1)  $foo = 1 + "10 Small Pigs"; // $foo is an integer (11) 　
$foo = 1 + "10 Little Piggies"; // $foo is a double (11); the string contains 'e'
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