Variables

Variables in PHP start with a $ symbol followed by the variable name. Variable names must begin with a letter or underscore, and can only contain letters, digits, and underscores (A-z, 0-9, _). They are case-sensitive ($y and $Y are different variables). The var_dump() function outputs the type and value of a variable, commonly used for debugging.

Example:

$mumu = 1111;
$didi = 2222;
$didi =& $mumu; // Reference assignment
$mumu = 8888;
echo $didi; // Output: 8888

This behavior mimics shared data — when $didi references $mumu, changing $mumu also affects $didi.

Variable Assignment Types:

• Direct Copy: $new_name = $name;
• Reference Assignment: $new_name2 = &$name; — changes to one affect the other.
• Variable Variables: Using $$$name to dynamically access variables based on string values.

Constants

Constants are defined using define() and cannot be changed after definition. They do not use $ and are typically written in uppercase for clarity.

define('NAME', '赵六');
echo NAME; // Output: 赵六

Use defined("NAME") to check if a constant exists.

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Data Types

PHP supports three main categories:

1. Scalar Types:

   • int: integers (e.g., 100)
   • float: floating-point numbers (e.g., 100.24)
   • string: text (e.g., "Hello")
   • bool: boolean (true/false)

2. Compound Types:

   • array: ordered collections (e.g., [1, 2, 3])
   • object: instances of classes (e.g., new People())

3. Special Types:

   • null: represents no value
   • resource: handles external resources (e.g., file pointers)

Example:

$arr = [1, 3, 4];
var_dump($arr);

class People {
    public $name = "123";
}
$peo = new People();
var_dump($peo);

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Type Conversion and Operators

Implicit Type Casting

PHP automatically converts types during operations:

$num = 100;
$float_val = 20.01;
var_dump($num + $float_val); // float(120.01)

// String to int: stops at first non-digit
$str = "100.01aaaa";
var_dump($str + 10); // int(110)

Explicit Type Casting

Use functions like intval(), floatval(), and boolval():

$str = "101abc";
var_dump(intval($str));     // int(101)
var_dump(floatval($str));   // float(101)
var_dump(boolval($str));    // bool(true) (non-empty string)

Type Checking Functions

var_dump(is_bool(true));      // true
var_dump(is_string("abc"));  // true
var_dump(is_array([1]));      // true

Operators

• Arithmetic: +, -, *, /, %, ++, --
• String Concatenation: ., e.g., $name . "" . $age
• Assignment: +=, -=, *=, /=, %=
• Comparison: ==, ===, !=, !==, <, >, <=, >=
• Logical: and, or, !
• Ternary: condition ? true_value : false_value

Example:

echo 100 > 50 ? "yes" : "no"; // Output: yes

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Control Flow

Sequential Execution

echo "1";
echo "2";
echo "3";

Conditional Branching

if (1 < 2) {
    echo "Expression is true";
} else {
    echo "False";
}

$age = 17;
if ($age >= 18) {
    echo "You can play freely";
} else {
    echo "Underage – limit gameplay to 2 hours";
}

Multi-way Branching

$age = 35;
if ($age < 18) {
    echo "Underage";
} elseif ($age <= 30) {
    echo "Play freely";
} elseif ($age <= 50) {
    echo "Consider spending money";
} else {
    echo "Senior, take care of your health";
}

Loops

• For Loop:

for ($i = 0; $i < 10; $i++) {
    echo $i . "<br>";
}

• While Loop:

$num = 1;
while ($num <= 10) {
    $num++;
    if ($num == 5) continue; // Skip iteration
    echo $num;
}

• Do-While Loop:

$num = 1;
do {
    echo "Loop body";
    echo $num;
} while ($num > 10); // Executes at least once

• Foreach Loop (for arrays):

$arr = ['a', 'b', 'c'];
foreach ($arr as $key => $value) {
    echo "Key: $key, Value: $value<br>";
}

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Functions

Functions are reusable blocks of code.

Function Definition

function greet($name) {
    echo "Hello, $name!<br>";
}

// Call function
greet("Alice"); // Output: Hello, Alice!

Parameters and Return Values

• Parameters: Passed by value (copy), unless referenced with &
• Return Values: Use return to send back results

function add($a, $b) {
    return $a + $b;
}
$result = add(5, 3);
var_dump($result); // int(8)

Default Parameters

function greet($name, $greeting = "Hi") {
    echo "$greeting, $name!<br>";
}
greet("Bob");        // Hi, Bob!
greet("Charlie", "Hey"); // Hey, Charlie!

Scope and Global Variables

• Local Variables: Defined inside a function
• Global Variables: Accessible via global keyword

$name = "Tom";
function showName() {
    global $name;
    echo $name;
}
showName(); // Output: Tom

Static Variables

Persist between function calls:

function counter() {
    static $count = 0;
    $count++;
    echo $count . "<br>";
}

counter(); // 1
counter(); // 2

Variable Functions

Functions can be called dynamically using variable names:

$func_name = "counter";
$func_name(); // Calls counter()

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Arrays

Arrays store multiple values under a single variable.

Indexed Array

$arr = array('dong', '18', '网络安全专家');
echo $arr[2]; // Output: 网络安全专家

Associative Array

$arr = array(
    'name' => 'dong',
    'age' => '18',
    'profession' => '网络安全专家'
);

foreach ($arr as $key => $value) {
    echo "Key: $key, Value: $value<br>";
}

Multidimensional Array

$arr = array(
    'name' => 'dong',
    'age' => '18',
    'skills' => array('网络安全', '网络工程')
);

foreach ($arr as $key => $value) {
    if (is_array($value)) {
        foreach ($value as $sub_key => $sub_value) {
            echo "Sub-array: $sub_value<br>";
        }
    } else {
        echo "Key: $key, Value: $value<br>";
    }
}

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Timestamps

Convert time to Unix timestamp and back:

$timestamp = time();
echo $timestamp . "<hr>";
echo date('Y-m-d H:i:s', $timestamp); // Output: 2020-06-10 08:33:21

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GET and POST Requests

• GET: Used for retrieving data (e.g., http://example.com/page.php?name=dong)
• POST: Used for sending data (e.g., form submissions)

Access data via superglobals:

var_dump($_GET); // Get query parameters
$l = $_GET['mumu'];
file_put_contents("./22.txt", $l); // Save to file

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Ternary Operator

Short-hand conditional expression:

$a = 1 ? "2343" : "300"; // Result: "2343"

// Safe default handling
$b = isset($_GET['name']) ? $_GET['name'] : "default";

Use empty() to check for empty values, isset() to check existance.