Introduction to comments, variables, arrays, constants, and function applications in PHP

What is the difference between single quotes and double quotes in php?
1. As can be seen from the following, variables with double quotes are parsed and output, while variables with single quotes are not parsed.
2. The parsing speed of single quotes is faster than that of double quotes
3. For single quotes, there are only two escapes ',\
4. Except for the above two escape characters, everything else is the same output.
5. For double quotes, in addition to \, ', ", there are also t, r, n, etc. that can be escaped.
6. In addition to single quotes and double quotes used to declare string variables , and heredoc method
program code:
//$age = 22;
//$str1 = 'He is $age this year'; //''Output as is
//$str2 = "He is $age this year"; //""To parse the variables inside, output 22
//echo $str1,"
",$str2;
Comments in php
(1) Start with //.
(2) Start with #.
#This is the shell single-line comment style
(3) Multi-line There is a kind of comment that starts with /* and ends with */
Notes in PHP
1. In js, if a statement occupies a single line, there is no need to add a ; sign at the end of the line
2. But , in PHP, a semicolon must be added after each line
Although there is an exception in the last sentence of the entire PHP, it is strongly recommended to add
3. For pure PHP pages, ?> does not need to be written
Moreover, for files that are not run directly but are included in other pages, it is often recommended to end them without adding ?>
The pages included in this way will execute faster and faster
php Variables in
1. There are 8 types of PHP variables
2. Integer type, floating point type, Boolean type, string type, NULL type, array type, object type, resource type
3 . In js, declare variables with var variable name [= value]. To declare variables in php, directly variable name = value;
4. Variable naming convention in php
5. The name of the variable consists of "letters" ", underscores, numbers, and combinations. And numbers cannot begin
6. There is a '$' mark in front of variables in PHP
7. echo is not a function, but a grammatical structure.
8 . You can print out variables
9. When you want to print multiple variables, separate them with ','
10. Passing values of variables
11. Passing by value and assignment
15. String type
Variables and constants
(1) Constants
Variables can be reassigned at any time
//$age = 22;
//$age = 23;
//echo "
",$age;
(2) What is the difference between variables and constants?
1. Their declaration methods are different
2. Once a constant is declared, Its value cannot be changed
define('PI',3.14);
PI =3.23; Syntax error
echo "
",PI;
3. Variables can be destroyed, and Once a constant is established, it cannot be destroyed
unset($age);
var_dump($age);
4. Constants cannot be logged out
unset(PI); //Syntax error, logout is not allowed
echo PI;
5. Variables have their own scope, and external variables cannot be accessed by default inside the function.
And constants, once defined, are defined either globally or within the function.
Can be accessed anywhere on the page.
(3) Naming conventions for variables and constants
1. The naming conventions for constants are the same as those for variables from a grammatical perspective.
A combination of letters, numbers, and underscores is allowed, and numbers cannot begin with
2. From a customary perspective: Generally, constants are in "uppercase"
//define('SF',342);
//echo SF; //The latest version has made constant names case-sensitive
(4) What are the allowed values of constants?
1. Only scalar types (single types) can be assigned to constants;
2. Composite types, such as arrays and objects, cannot be assigned to a constant.
3. Resource type If assigned to a constant, it may cause some unpredictable errors.
The code is as follows:

Copy the code The code is as follows:

 
define('AGE' ,22); 
define('HEI',343.234); 
define('ROOT','D:/www'); 
define('LOCK',true); 
define('NON',NULL); 
echo AGE,HEI,ROOT; 
var_dump(LOCK); 
var_dump(NON); 

Control structure in PHP
(1) Any program is inseparable from variables, expressions, and control structures
(2) In PHP, else if can be connected Writing is not allowed in js. We recommend standard writing, that is, esle if separation.
(3) In PHP, the scope of variables does not look out along the scope like in JS.
(4) In PHP, there is a special type of variable called super global variable. No matter you are in a function or inside a class, no matter how deep the code is packaged
you can access the variable.
php arrays and js arrays
(1) Two ways to create arrays in js
(2) In js, the index of an array always starts from 0 and increases one by one, with no gap in between
1 . var arr=new Array(1,2,3,4);
2. var arr= [1,2,3]
(3) Create array in php
1. But in php , the index of the array is very flexible
2. It can be a number or a string
3. It can even be a mixture of numbers and strings
4. If the index part specifies a numeric index
5. There is also a unit without a specified index
6. Then take the largest numeric index value that has appeared before in this unit and then +1 as its key value
php creates an array as follows:

Copy code The code is as follows:

 
$arr=array(1,2,3); 
print_r($arr); 
 //============================================ 
 $arr=array(10=>'Zhao','adfdssd'=>'Qian','Sun','name'=>'Zhang Sanfeng'); 
print_r($arr); 
 

7. In PHP, how to reference the cell value of an array depends on the index
8. And the index is a numeric index
//echo $arr[10];
9. If It is a string index, and single quotes must be added. If no single quotes are added, it will be treated as a constant first
//define('name','adfdssd');
//echo $arr[name] ;
(4) Associative array and index array
1. The index may be a pure number, a string, or a mixed string + number
2. If the index is a pure number, It is called 'index array';
3. Otherwise it is called "associative array";
(5) The difference between functions in php and functions in js
1. In js, you can have more Declaring a function with the same name multiple times
2. But in a PHP page, a function with the same name cannot be used multiple times
3. In JS, function names are case-sensitive
4. In PHP, function names are not case-sensitive (Class methods are not distinguished)
5. In PHP, the number of parameters when calling a function must be consistent with the parameters of the declared function
6. In PHP functions, when the function is declared, a certain parameter can have " Default value"
Code display for all the above knowledge points

Copy the code The code is as follows:

 
//====================================== ============= Return to original location
//2. Integer type, floating point type, Boolean type, string type, NULL type 
$age = 22; 
 $weight = 75.23; 
$name ='Zhang San'; 
$money = false; 
$house = null; //Equivalent to undiffed in js 
echo $age,$weight, $money, 
//============================================ ====== Return to the original place
//10. Value transfer of variables
$age =22; 
$nian =$age;//Read the value of $age and assign it to $nian 
$nian= 24; 
echo $nian,'----',$age; 
//================== ============================== 
//11. Pass by reference, assignment and pass by value 
$money =10000 ; 
$credit = &$money; //Declare the $credit variable and point the credit pointer to the storage space of money 
$credit = 5000; 
echo $credit,'------- -',$money; 
unset($credit); 
echo $credit; 
//======================== ========================= 
//String type
$str1 = 'hello'; 
$str2 = " world"; 
echo $str1,$str2,"
"; 
//======================== ========================= 
$age = 22; 
function t(){ 
var_dump($age); 
} 
t(); 
define('HEI',88.63); 
function s(){ 
var_dump(HEI); 
} 
s(); 
// Note: Functions in php cannot be declared repeatedly. Variables in functions are packaged more strictly and only work within the function. It will not run outside and work 
//5. For the above situation, you can use variables or constants, but we choose constants. 
//Reasons: The first is ROOt, which is often quoted 
//The second is: if a variable is used, $ROOT ='a'; it is very likely that the value will be changed during multi-person development 
// There are also disadvantages to using constants: 
//Once a constant is defined, it will not be destroyed 
//Constant is always internal and cannot be destroyed. 
//================================================ ======== 
//In php, variable names are also variable. 
$talk='hello'; 
$heat= 'kill you'; 
$love= 'love'; 
echo $love,"
"; 
 $action = 'talk'; 
$t ='action'; 
echo $$$t; 
//==================== ================================== 
//Advance notice: not only variable names, but also function names can be Changeable, the class name is also variable. 
////Constant names also need to be changed 
define('PI',3.14); 
define('HEI',342); 
$cons= 'PI'; 
echo $cons,"
"; 
echo constant($cons); //constant is the name that treats the value of the variable as a constant, referencing the constant
//====== =============================================== Return to original position 
//Control structure in php
//Any program is inseparable from variables, expressions, and control structures
if ,if/else,if/else if/ esle 
$num =3; 
if($num >2){ 
echo 'In php, 3 is also greater than 2',"
"; 
} 
if($num >5){ 
echo '3 is greater than 5',"
"; 
}else{ 
echo '3 is not greater than 5'; 
} 
// 
if($nun==1){ 
echo 'Today is Monday'; 
}else if($num ==3){ 
echo 'Today is Wednesday'; 
 }else{ 
echo 'not one, not two, not three'; 
} 
////In php, else if can be written consecutively, but it is not allowed in js
//We recommend Standard writing, that is, esle if separated. 
//================================================ ======== 
switch case statement 
$num = 3; 
switch($num){ 
case 1: 
echo 'Today is Monday',"< br />"; 
break; 
case 2: 
echo 'Today is Tuesday',"
"; 
break; 
case 3: 
echo 'Today is Wednesday',"
"; 
break; 
default: 
echo 'Don't know'; 
break; 
} 
/* 
Suppose someone has 100,000 in cash and needs to pay a fee every time he passes through an intersection. 
The tariff rule is that when his cash is greater than 50,000, he needs to pay 5% in cash every time he passes through the intersection. If his cash is less than or equal to 
= 50,000, he needs to pay 5,000 each time. Please write a program to calculate how many times this person can pass by. This intersection
*/ 
for ($m =100000,$num=0;$m>=5000;$num++){ 
if ($m>50000){ 
$m*= 0.98; 
}else{ 
$m-=5000; 
} 
} 
//==================== ======================= 
//while ,do/while 
//Use while to print $1-9; 
$i =1; 
while($i<10){ 
echo $i++,"
"; 
} 
$i=0; 
while(++$ i<10){ 
echo $i,"
"; 
} 
//==================== ======================= 
while(){} 
$i=0; 
while(++$i<10 ){ 
if($i==5){ 
break; 
continue; 
} 
echo $i,"
"; 
} 
////============================================ 
//Super global variables in php 
$num =99; 
function t(){ 
echo $num; 
//} 
////In this calling process, $num is null because $num 
//// is not defined inside the function and in php, it does not search outside the scope like js. 
t(); 
///======================================== ====== 
//In php, there is a special type of variables called super global variables. 
//Whether you are in a function or inside a class, no matter how deep the code is packaged 
//You can access this variable.
function a(){ 
echo $_GET['title']; 
} 
a(); 
///============= ============================== 
//I want to use php to make a guestbook
//About logical operators 
var age= 2||3; 
alert(age); 
var_dump($age); 
//In php, logical operations return true/false 
$age = 2 ||3; 
var_dump($age); 
$a = 3; 
$b= 2; 
if($a=9 || $b=1){ 
$ a +=1; 
$b +=1; 
} 
echo $a,"
",$b; 
//Function in php and js The difference between functions
//1. In js, you can declare a function with the same name multiple times
//But in a php page, you cannot declare a function with the same name multiple times
//2. In js, Function names are case-sensitive
//In php, function names are not case-sensitive (class methods are not case-sensitive either) 
///================== ========================== Return 
///3. In PHP, the number of parameters when calling a function must be the same as the number of parameters when declaring the function. The parameters are consistent 
$a = 1; 
$b = 2; 
$c = 3; 
function t($a,$b,$c){ 
echo $a+$b+ $c; 
} 
t(1,2,3); 
t(1,2); 
///================ ============================ 
//4. In PHP functions, when the function is declared, a certain parameter can have " Default value" 
function t($a,$b,$c=0){ 
echo $a+$b+$c; 
} 
t(1,2); 
? > 

Introduction to comments, variables, arrays, constants, and function applications in PHP_PHP Tutorial