How to make an array retain only key names in php

Two implementation methods: 1. Use the array_keys() function to obtain all key names of the array, with the syntax "array_keys (array)"; if you want to retain the key names of the specified value, you can set the second and third Parameters, syntax "array_keys (array, specified value, type is consistent)". 2. Use a foreach loop and an empty array to obtain all the key names of the array. The syntax is "foreach($arr1 as $k=>$v){$arr2[]=$k;}".

The operating environment of this tutorial: windows7 system, PHP version 8.1, DELL G3 computer

php Two ways to make the array only retain key names

Method 1: Use the array_keys() function to get all the key names of the array

array_keys() The function can retrieve some or all keys in the array.

array_keys(array,value,strict)

Parameters	Description
array	Required . Specifies an array.
value	Optional. You can specify a key value, and then only the key name corresponding to that key value will be returned.
strict	Optional. Used with the value parameter. Possible values: true - Returns the key name with the specified key value. Depending on the type, the number 5 is not the same as the string "5". false - the default value. Independent of type, the number 5 is the same as the string "5".

When you want to get all key names, just set the first parameter and omit the second and third parameters.

Example:

<?php
header("Content-type:text/html;charset=utf-8");
$arr=array("Name"=>"Peter","Age"=>"41","Country"=>"USA");
var_dump($arr);
$keys=array_keys($arr);
echo "数组只保留键名:";
var_dump($keys);
?>

If you want to get the key name of the specified value, the second and third parameters are not omitted

<?php
header("Content-type:text/html;charset=utf-8");
$arr=array("Name"=>"Peter","Age1"=>"41","Age2"=>41,"Country"=>"USA");
var_dump($arr);
$keys1=array_keys($arr,41);
echo "数组只保留键名:";
var_dump($keys1);
$keys2=array_keys($arr,41,true);
var_dump($keys2);
?>

Method 2: Use a foreach loop and an empty array to get all the key names of the array

Implementation idea:

Use a foreach loop to traverse the key names and key values ​​​​of the original array, and only assign the key names to the empty array.

Example:

<?php
header("Content-type:text/html;charset=utf-8");
$arr1=array("aaa"=>11,"bbb"=>22,"ccc"=>33);
var_dump($arr1);
$arr2=array();
foreach($arr1 as $k=>$v){
    $arr2[]=$k;
}
echo "数组只保留键名:";
var_dump($arr2);
?>

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