PHP (PHP: Hypertext Preprocessor) is a widely-used open-source scripting language. PHP files can contain text, HTML, JavaScript, and PHP code. PHP code executes on the server, and results return to browsers as plain HTML. The default file extension for PHP files is .php.

PHP scripts start with <?php and end with ?>. Variables start with a $ symbol, followed by the name. Variable names must begin with a letter or underscore, only include alphanumeric characters and underscores, and cannot contain spaces.

<?php
    $greeting = "Hello PHP";
    echo $greeting;
?>

PHP supports several data types: String, Integer, Float, Boolean, Array, Object, and NULL. Use var_dump() to check data types.

<?php
$stringVal = "Text data type";
$intVal = 5678;
$floatVal = 3.14159;
$expVal = 2E4;
$boolVal = false;
$arrVal = array("X", "Y", "Z");
class SampleObject {
    var $property;
    function method() {}
}
$objVal = new SampleObject();
$nullVal = NULL;
var_dump($stringVal, $intVal, $floatVal, $expVal, $boolVal, $arrVal, $objVal, $nullVal);
?>

Constants are fixed values that can't change in a script. Define them with define(), which takes three parameters: name, value, and optional case-insensitivity flag.

<?php
define("APP_TITLE", "Global Application Title", true);
echo APP_TITLE;
echo app_title;
?>

PHP uses . to concatenate strings. Key string functions include strlen() (length), strpos() (find position), str_replace() (replace text), and trim() (remove whitespace).

<?php
$str1 = "good";
$str2 = "morning";
echo $str1 . " " . $str2 . "<br>";
echo "Length of str1: " . strlen($str1) . "<br>";
echo "Position of 'o' in str1: " . strpos($str1, 'o') . "<br>";
$modified = str_replace('o', 'a', $str1);
echo "Modified str1: " . $modified . "<br>";
$whitespaceStr = "   Trim this string   ";
echo "Trimmed string: " . trim($whitespaceStr);
?>

Control statements include conditional statements (if, if...else, if...elseif...else, switch) and loops (for, while, do...while, foreach). break terminates a loop, continue skips to the next iteration.

<?php
$score = 85;
if ($score >= 90) {
    echo "Excellent";
} elseif ($score >= 80) {
    echo "Good";
} else {
    echo "Needs improvement";
}
$numbers = array(1, 2, 3, 4, 5);
foreach ($numbers as $num) {
    if ($num == 3) continue;
    echo "Number: " . $num . "<br>";
}
?>

Define custom functions with function keyword. Functions can take parameters, have default parameter values, and return values. Use return to output results.

<?php
function greetUser($name = "Guest") {
    return "Hello, " . $name . "!";
}
echo greetUser("Alice") . "<br>";
echo greetUser();
?>

Variablee scope in PHP includes local (function-level), global (script-level), and static (persistent local variables). Use global to access global variables inside functions, and static to preserve variables after function execution.

<?php
$globalVar = 10;
function testScope() {
    global $globalVar;
    static $count = 0;
    $count++;
    echo "Count: " . $count . "<br>";
    echo "Global var: " . $globalVar . "<br>";
}
testScope();
testScope();
testScope();
?>