Indexed array:
An indexed array is an array that uses a numeric index to access its elements. The index is a number that starts at 0 for the first element and increases by 1 for each subsequent element.
We can create an indexed array in PHP,
(i) by enclosing a comma-separated list of values in square brackets (or)
(ii) by using the array() function.
Syntax
$array Variable=[element1, element2, element 3, ... elementN];
(or)
$arrayVariable array( element1, element2, element3.... elementN);
Examples
(a) $fruits = ['apple, "banana, 'orange'];
(b) $fruits = array('apple, "banana, 'orange');



Associative array
An associative array is a data structure that store a collection of key-value pairs. The keys are used to identify the values, and the values can be of any data type. The keys in an associative array are often called "labels" because they label or identify the corresponding values.
We can create an associative array in PHP,
(i) by enclosing a comma-separated list of key-value pairs in square brackets (or) 
(ii) by using the array() function.
Syntax

Example
$student [
"Name" => "Ram",
"Age" => 25,
"Place" => "Trichy"
];

Array is a concept that stores more than one value of same data type in single array variable.
They are 3 types of array concepts in PHP.
(i) Indexed Arrays
(ii) Associative Array and
(iii) Multi-Dimensional Array
(i) Indexed Arrays
Array defines with the keyword array()
Syntax:

Array with numeric index for the available values in array variable which contains key value pair as user can take the value using keys.
Example:
$teacher_name=array("Iniyan", "Kavin", "Nilani");
echo "The students name are".$teacher_name[0].$teacher_name[1]..
(ii) Associative Arrays
Associative arrays are a key-value pair data structure.
Syntax:
array (key=> value, key =>, value, key => Value, etc.);
key=Specifies the key (numeric or string)
Valme - Specifies the value
Example
<$ php
$Marks
array"Student1"=>"35","Student2"=>"17"Student3"=>"43");
echo "Studenti mark is. $Marks("Student1]." is eligible for qualification";
echo "Student2 mark is. $Marks Student2"]." is not eligible for qualification";
?>
(iii) Multidimensional Arrays:
Array containing one or more arrays is called multidimensional array. PHP accepts array with two, three four or more levels.
Example:
< ?php
// A two-dimensional array
$student=array
(
array("Iniyan", 100, 96),
array("Kavin", 60, 59),
array("Nilani", 1313, 139)
);
echo ＄＄student [0][0].": Tamil Mark: ".＄student [0][1].". English mark: ".＄student [0] [2]."< br > ";
echo ＄＄student [1][0].": Tamil Mark: ".＄student [1][1].". English mark: ".＄student [1] [2]."< br > ";
echo ＄＄student [2}[0].": Tamil Mark:".＄student [2][1].".English mark: ".＄student [2] [2]."< br >
? >

Multidimensional Arrays:
Array containing one or more arrays is called multidimensional array. PHP accepts array with two, three four or more levels.
Example:
// A two-dimensional array
$student=array
(
array("Iniyan", 100, 96),
array("Kavin", 60, 59),
array("Nilani", 1313, 139)
);
echo ＄＄student [0][0].": Tamil Mark: ".＄student [0][1].". English mark: ".＄student [0] [2]."< br >";
echo ＄＄student [1][0].": Tamil Mark: ".＄student [1][1].". English mark: ".＄student [1] [2]."< br >";
echo ＄＄student [2}[0].": Tamil Mark:".＄student [2][1].".English mark: ".＄student [2] [2]."< br >
? >

1. User Defined Function (UDF) in PHP givesa privilege to user to write own specific operation inside of existing program module.
2. Two important steps the Programmer has to create for users define Functions are:
Function Declaration
1. User-defined Function declaration begins with the keyword "function".
2. User can write any custom logic inside the function block.
Syntax:
function functionName()
{
Custom Logic code to be executed;
}
Function Calling: 
1. A function declaration part will be executed by a call to the function.
2. Programmer has to create Function Calling part inside the respective program.
Syntax:
functionName();
Example:
function insertMsg() {
echo "Student Details Inserted Successfully!";
}
insertMsg(); // call the function
?>

Function in PHP:
A block of segment in a program that performs a specific operation such as Insert, Execute, Delete, Calculate etc.
A function is a type of sub-routine or procedure in a program.
The function can be divided into three types as follows.
(i) User defined Function
(ii) Pre-defined or System defined or Built-in Function
(iii) Parameterized Function
(i) User defined Function
PHP gives a privilege to user to write own specific operation inside of existing program module.
Two important steps the programmer has to create for users defined functions are
(a) Function Declaration
(b) Function Calling
(a) Function Declaration
A user defined function declaration begins with the keyword "Function".
User can write any custom logic inside in function block.
Syntax:
function functionName()
{
custom logic code to be executed;
}
(b) Function Calling
A function declaration part will be executed by a call to the function.
Programmer has to create function calling part inside the respective program.
Syntax:
functionName();
(ii) Pre-defined or System defined or built-in Function
Built-in functions are functions that exist in PHP installation package.
Functions can either return values when called or can simply perform an operation without returning any value.
Example:
rename_function()
This function renames original name to new name.
Parameter is also called as arguments just like variables.
Arguments are mentioned after the function name and inside the parenthesis.
There is no limit for sending arguments, just separate them with a comma notation.
Examples :
Parameterized function with one argument, two arguments respectively.
(i) function School Name ($sname)
(ii) function School Name ($sname, $strength)