PHP 5.2.x details not to be noticed: array pieces

Just read the PHP tutorial, due to the PHP version of the problem found that there is a place to study the array here
Several experiments were made for php5.2.5:

1,

$arr   =   Array ( " a "   =>   1 , " b "   =>   2 , " C "   =>   3 );

However, if you define an array like this, you will report a compilation error:

$arr   =   Array ( " a "   =   1 , " b "   =   2 , " C "   =   3 );

So when you define an array, you can only use =>

2,

$arr   =   Array ( " a "   =>   1 , " b "   =>   2 , " C "   =>   3 );
Echo   $arr [ 0 ];
Echo   $arr [ 1 ];

It is a blank, so that it can be played out:

Echo   $arr [ " a " ];

3, add elements or modify elements of the time can only be used =, can not use =>

$arr   =   Array ( " a "   =>   1 , " b "   =>   2 , " C "   =>   3 );
$arr [ " C " ]  =>   6 ;

Like this may be used in previous releases, but there will be a compilation error when 5.2.5
Add elements or modify elements to write as follows:

$arr   =   Array ( " a "   =>   1 , " b "   =>   2 , " C "   =>   3 );
$arr [ " D " ]  =   4 ;
$arr [ " C " ]  =   6 ;
Delete element to use unset


unset  ( $arr [ " C " ]);

4. Do an experiment and guess what it is:

$arr   =   Array ( " a "   =>   1 , 2 , " b "   =>   3 , 4 );
$arr []  =   5 ;
foreach ( $arr   as   $key   =>   $value )
{
Echo   " key: $key value: $value <br> " ;
}

Results:

Key:a value:1
key:0 Value:2
Key:b Value:3
Key:1 Value:4
Key:2 Value:5

This makes it clear that PHP will automatically use a number starting with 0 as a key when the user does not have a key defined.

5, the array in PHP has a pointer, you can set the group forward and backward operation

$arr   =   Array ( " a "   =>   1 , 3 , " b "   =>   2 );
// After you create the array, the default pointer refers to the first element
Echo   Current ( $arr ) . " <br> " ;
// forward one position
Echo   Next ( $arr ) . " <br> " ;
// The default principle of collation is from small to large
Sort ( $arr );
// after finishing, the array pointer stops at the first element
Echo   Current ( $arr ) . " <br> " ;
Echo   Next ( $arr ) . " <br> " ;
// back one position
Echo   prev ( $arr ) . " <br> " ;

Output:

1
3
1
2
1