what is explode function in php?

You have all explained on this link, there you have a lot of examples that will show you everything.
Basicly it is a function that split sentence (any string) into array of words (string), but you need to send a request how to split that sentence.
For example:
"What + are + you + doing?"
For split u use: " + ";
You will get array like this:
Position 0 = "What";
Position 1 = "are";
Position 2 = "you";
Position 3 = "doing?";
Array start from 0 if you wonder what it is position 0.
Regards, Mike.

To add to what Mike has said, I' d like to share the syntax with an example-
$a=explode('2014-12-20','-');

the code snippet would return an array assigned to variable $a as follows:

$a[0]="2014", $a[1]="12", $a[2]="20"

To add to what JoyBh said, I'd like to share the correction. ;)

explode('-', '2014-12-20');

// returns

Array
(
    [0] => "2014",
    [1] => "12",
    [2] => "20"
)

Syntax is explode([delimiter], [string]);

To add more... explode() is the newer/safer version of split(). However, split() has been deprecated since version 5.3.0.

Explode() is a construct developed for the lemmings game, it makes the lemming explode to blow a hole through the wall or floor
similarly
float() is a construct developed for the lemmings game, it makes the lemming move slowly down,

The PHP function explode lets you take a string and blow it up into smaller pieces. For example, if you had a sentence you could ask explode to use the sentence's spaces " " as dynamite and it would blow up the sentence into separate words, which would be stored in an array. The sentence "Hello, I would like to lose weight." would look like this after explode got done with it:

Hello,
I
would
like
to
lose
weight.

The dynamite (the space character) disappears, but the other stuff remains, but in pieces. With that abstract picture of the explode function in mind, lets take a look at how it really works.
The explode Function

The first argument that explode takes is the delimiter (our dynamite) which is used to blow up the second argument, the original string. explode returns an array of string pieces from the original and they are numbered in order, starting from 0. Lets take a phone number in the form ###-###-#### and use a hyphen "-" as our dynamite to split the string into three separate chunks.
PHP Code:

$rawPhoneNumber = "800-555-5555";

$phoneChunks = explode("-", $rawPhoneNumber);
echo "Raw Phone Number = $rawPhoneNumber <br />";
echo "First chunk = $phoneChunks[0]<br />";
echo "Second chunk = $phoneChunks[1]<br />";
echo "Third Chunk chunk = $phoneChunks[2]";

Display:
Raw Phone Number = 800-555-5555
First chunk = 800
Second chunk = 555
Third Chunk chunk = 5555