Can php array key values ​​be repeated?

Arrays in PHP are a very common data structure that allow you to store a set of data together and access the data using different types of key values ​​such as numbers or strings. When working with PHP arrays, you may encounter a question: is it possible to store multiple values ​​using the same key? This article will answer this question and introduce some related concepts and techniques.

First of all, it needs to be clear that PHP's array is an associative array (associative array), which can map key values ​​to any type of value. This means that each key can only correspond to one value. If you try to store multiple values ​​with the same key, the new value will overwrite the old value. For example:

$fruits = array(
  'apple' => 'red',
  'banana' => 'yellow',
  'apple' => 'green'
);

echo $fruits['apple']; // 输出 green

In the above example, when we add 'apple' => 'green' to the $fruits array, it overwrites the old 'apple' => 'red'. So when we print $fruits['apple'] it will return 'green', not 'red'.

This is because, in PHP, if you add a new element using the same string key, the new element will replace the old element. This is because PHP treats the keys of the array as keys in a hash table, and if the hash code of the new value is the same as the hash code of the old value, the new value will replace the old value. This also means that if you try to add elements repeatedly using integer keys, the new elements will be appended to the end of the array instead of replacing the old values.

In addition to the string keys used in the examples above, PHP also supports other types of keys, including integers, floating point numbers, Boolean values, and objects. Since PHP converts all keys to strings, you may run into some problems if you try to add elements with different types of keys. For example:

$fruits = array(
  'apple' => 'red',
  0 => 'hello',
  1.5 => 'world',
  true => 'yes',
  new stdClass() => 'object'
);

print_r($fruits);

In the above example, we are trying to add elements using different types of keys like integer, float, boolean and object. However, if you run the above code, you will see that all elements are added to the array, but the keys are converted to strings. For example, the integer 0 will be converted to the string '0', the float 1.5 will be converted to the string '1.5', the boolean true will be converted to the string '1', and the object will be converted to the string ' [object]stdClass'.

In some cases, you may need to store multiple values ​​with duplicate keys. In order to achieve this function, you can use another data structure of PHP - multidimensional array (multidimensional array). Multidimensional arrays can nest multiple arrays together, and each array can have keys of the same type.

For example, the following example demonstrates how to use a multidimensional array to store multiple colors:

$fruits = array(
  'apple' => array('red', 'green'),
  'banana' => array('yellow')
);

print_r($fruits);

In the above example, we created a $fruits array with two elements, where each Each element is an array used to store the color of the fruit. Now, we can access a specific color by using a key, for example:

echo $fruits['apple'][0]; // 输出 red
echo $fruits['apple'][1]; // 输出 green
echo $fruits['banana'][0]; // 输出 yellow

When using multi-dimensional arrays, we need to be aware that storing multiple values ​​by using nested arrays will increase the complexity of the code , and may make the code harder to understand and maintain. Therefore, if you are able to use other data structures to store multiple values, you should avoid using duplicate keys whenever possible.

In summary, PHP's array is an associative array, which does not allow storing multiple values ​​with the same key. If you need to store multiple values, you might consider using a multidimensional array or other data structure to do so. At the same time, when you use different types of keys, you need to note that PHP will automatically convert the keys into strings, which may cause some unnecessary errors and problems.

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