how do I use Json_decode in php?

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PHP

Hi everyone

can any one tell me how do I use Json_decode in php. I have simple json file
I couldn't use it to extract all data.

see this

$decode =json_decode('{
        "worldpopulation":
	    [
		 {
		 "rank":1,"country":"China",
		 "population":"1,354,040,000",
		 "flag":"http://www.androidbegin.com/tutorial/flag/china.png"
		 },

		 {
		 "rank":2,"country":"India",
		 "population":"1,210,193,422",
		 "flag":"http://www.androidbegin.com/tutorial/flag/india.png"
		 },

		 {
		 "rank":3,"country":"United States",
		 "population":"315,761,000",
		 "flag":"http://www.androidbegin.com/tutorial/flag/unitedstates.png"
		 }
	]
}');

I want to extract all data?

Sorry for English