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Php has not only indexed arrays, but also concatenated arrays. PHP arrays can be divided into two types: 1, an array with numbers as its key name, that is, an indexed array; and 2, an array with a string or a mixture of strings and numbers as its key name, namely an associative array. If the key name is a string, wrap the key name with a delimiter (single or double quotation marks).

Operating environment of this tutorial: windows7 system, PHP7.1 version, DELL G3 computer

Php has not only indexed arrays, but also concatenated arrays.

Each entity in the array contains two items, a key (key) and a value (value). The corresponding array elements can be obtained by key values, which can be numeric keys or associated keys. If a variable is a container for a single value, then an array is a container for multiple values.

An array can store different types of data and is a compound data type. The data structure is shown in the following figure:

In the PHP array, no matter what type of key name there is, there will be a value corresponding to it, that is, a key / value pair. Depending on the array key name data type, we can divide the PHP array into two types:

An array with a number as its key name, namely an indexed array (Indexed Array)

An array with a string or a mixture of strings and numbers as a key name, that is, an associative array (Associative Array).

1) indexed array

The subscript (key name) of an indexed array consists of numbers. By default, starting from 0, each number corresponds to the position of an array element in the array, which does not need to be specified. PHP automatically assigns an integer value to the key name of the indexed array, and then increments automatically from this value.

2) associative array

The subscript (key name) of an associative array consists of a mixture of numeric values and strings, and if an array has a key name that is not a number, then the array is an associative array.

If the key name is a string, wrap the key name with a delimiter-single quotation mark''or double quotation mark "".

Note: the key name cannot be NULL.

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