It is a collection of elements of the similar and dissimilar datatype, We can store element dynamically without providing size in PHP array.
base index of the array will 0 to size-1, for example, the array has five elements then the array index will be started from 0 to 4.
Advantage of Array:-
1)  We can store multiple elements using a single variable which is easy for program code.
2)  We can easily search and sort the elements  
Type of Array:- 
 1)  Index Array:-  it will store data using an index where the base index will be started from 0 to size-1.
$arrayname = array(12,23,34,11,56,67,78);
$arrayname = array("c",12,23.45,"hello");
$arrayname[0]=12
....

Example of Array:-
<?php
$arr = array(12,23,34,11,67,78);

for($i=0;$i<count($arr);$i++)
{
    echo $arr[$i],"<br>";
}
?>

Example of Array using Foreach Loop:-
<?php
$arr = array("C",1200,"CPP",3450,"DS",56700,79800,"welcome");
for($i=0;$i<count($arr);$i++)
{
echo $arr[$i]."<br>";
}

foreach ($arr as $arr1) {
echo $arr1,"<br>";
}
?>

Program To reverse array element in PHP?
<?php
$arr = array(12,23,34,11,67,78,88,98);
for($i=count($arr)-1;$i>=0;$i--)
{
    echo $arr[$i],"<br>";
}
?>
Program to calculate the sum of array elements in PHP?
<?php
$arr = array(12,23,34,11,67,78,88,98);

$s=0;
for($i=0;$i<count($arr);$i++)
{
    $s=$s+$arr[$i];
}
echo $s;
?>
 2)   Associative Array:-
 Using this we can store elements using key=>value pair where the key is used to provide identity and value is used to provide data.
for example, rno=>1001 here rno is key and 1001 is values.
$stu = array("rno"=>1001,"sname"=>"manish kumar","branch"=>"CS","fees"=>45000);
here rno,sname,branch and fees are the keys (=> association)  
Example of Associative Array:-
<?php
$stu = array("rno"=>1001,"sname"=>"manish kumar","branch"=>"CS","fees"=>45000);
foreach($stu as $key=>$value)
{
    echo "$key  is $value"."<br>";
}  
?>
.....................................................................................
Q1)  WAP to find max, second max element in an array?
<?php
$arr = array(12,20,23,11,21,17);
$max = $arr[0];
$smax= $arr[0];
for($i=1;$i<count($arr);$i++)
{
   if($max<$arr[$i]) //12>20
   {
  $smax=$max;
    $max= $arr[$i];
   }
   else if($smax<$arr[$i] && $smax!=$max)
   {
    $smax=$arr[$i];
   }
}
echo $max,"<br>",$smax;
?>
Q2)  WAP to sort the elements of the array in asc order?
Q3) WAP to calculate the sum of even and the odd number of elements?
3)  Mixed Array:-
  it is used to collect the similar and dissimilar types of elements using index and associative array.
$x = array(10,11,"rno"=>1001,"name"=>"manish kumar");
we can print mixed array using foreach loop.
Example of Mixed array in PHP:-
<?php
$arr = array(12,23,"rno"=>1001,"CPP","JAVA","fees"=>45000,true,12.345);
foreach ($arr as $key => $value) {
echo "key is $key and value is $value"."<br>";
}
?>

The PRedefine function of the array in PHP:-
print_r():-  It will display the structure of array elements
$arr = array(12,23,"hello","hi")
print_r($arr);
max():-   It will display maximum element in the numerical array.
$arr = array(12,23,34,45.67)
max($arr);
min():-   It will display minimum element in the numerical array.
$arr = array(12,23,34,45.67)
min($arr);
array_sum():-  It will display sum of array elements into a numeric array
$arr = array(12,23,34,45.67)
array_sum($arr)
sort():-  it will sort array elements in ascending order
$arr = array(12,23,34,45.67,2,3)
sort($arr)
print_r($arr)
in_array():-  it will check that element is exist in an array or not.it return a boolean value.
$arr = array(12,23,34,45.67,2,3)
if(in_array(12,$x))
{
 echo "found";
}
else
{
   echo "not found";
}
is_array():-  It will check that variable is normal or array type.
$x=10;
if(is_array($x))
{
    echo "array";
}
else
{
  echo "not array";
}
array_merge():-  it is used to merge two arrays in one array.
$a = array(10,11,12);
$b = array(45,67,89,10);
$c = array_merge($a,$b);
print_r($c);
array_intersect():-   It will display common elements in two array.
$a = array(10,11,12);
$b = array(45,67,89,10);
$c = array_intersect($a,$b);
print_r($c);
array_diff():-  It will display the first array element which not exist on second.
$a = array(10,11,12);
$b = array(45,67,89,10);
$c = array_diff($a,$b);
print_r($c);
array_unique():-  It will display unique elements only and repeated elements display once.
$d = array(2,3,4,2,7,9,11,90,3,4,4,4,5);
$d1 = array_uniqe($d);
print_r($d1);
array_splice():-  It is used to split array elements using an index.
$e = array_splice($d, 6);
print_r($e);
array_chunk():-   It is used to divide one dimension array to a multidimensional array. we will provide number of elements on each row.
$d = array(2,3,4,2,7,9,11,90,3,4,4,4,5);
$e1 = array_chunk($d, 6);
print_r($e1);
array_combine():-  It is used to convert index array to associative array but both arrays should be equal size.
$key = array("rno","sname");
$val = array(1001,"xyz");
$data = array_combine($key,$val);
print_r($data);
A complete example of array predefine functions in PHP:-
<?php
$x = array(12,23,34,11,67,78);
print_r($x);
echo max($x)."<br>";
echo min($x)."<br>";
echo array_sum($x)."<br>";
sort($x);
foreach($x as $y)
{
 echo $y."<br>";
}
if(in_array(121,$x))
{
 echo "found";
}
else
{
   echo "not found";
}
$x=array();
if(is_array($x))
{
    echo "array";
}
else
{
  echo "not array";
}
$a = array(10,11,12);
$b = array(45,67,89,10);
$c = array_merge($a,$b);
print_r($c);
$a = array(10,11,12);
$b = array(45,67,89,10);
$c = array_intersect($a,$b);
print_r($c);
$a = array(10,11,12);
$b = array(45,67,89,10);
$c = array_diff($a,$b);
print_r($c);
$d = array(2,3,4,2,7,9,11,90,3,4,4,4,5);
$d1 = array_unique($d);
print_r($d1);
$e = array_splice($d, 6);
print_r($e);
echo "<br>";
$d = array(2,3,4,2,7,9,11,90,3,4,4,4,5);
$e1 = array_chunk($d, 6);
print_r($e1);
$key = array("rno","sname");
$val = array(1001,"xyz");
$data = array_combine($key,$val);
print_r($data);
?>
ASSIGNMENT:-
Create a complete mark sheet program using array?
The solution to this program:-
<?php
$marks=array("phy"=>45,"chem"=>75,"maths"=>89,"eng"=>77,"hin"=>55);
$count=0;
$total=0;
foreach ($marks as $key => $value) {
if($value<33)
{
$count++;
}
else
{
$total=$total+$value;
}
}
if($count==0)
{ $per=$total/5;
if($per>=33 && $per<45)
    echo "Pass with third division";
    else if($per<60)
    echo "Pass with second division";
    else
    echo "Pass with first division";       
}
else if($count==1)
{
    echo "SUPPL";
}
else
{
   echo "Failed";
}
?>
WAP to display only unique elements in an array?
example   2,2,3,4,5,6,7,5   o/p   3 4 6 7
WAP to display repeated elements in array only once?
example  2,2,3,4,5,6,7,5   o/p   2 5
WAP to merge three arrays in one array?
WAP to sort only prime elements in an array?
WAP to find second max prime elements in the array?