what is a echo?

Answer 1

echo is used to print something on webpage

like-

echo "Hello world";
echo $x; // value of x will be printed

Answer 2

For more info
https://www.w3schools.com/php/php_echo_print.asp

Answer 3

Echo is the constant to output the result any type

Echo "hello world"
$hi="hello world"
Echo "$hi"

Answer 4

echo() = it can print one or more string , which may or may not enclosed with brackets.

Example :
echo "Green";
//or
echo("Green");

If you want to print multiple strings, it should use comma-separated,
and without enclosed in brackets.
Example :

echo "Red","Blue","Green"; // Valid statement

echo ("Red","Blue","Green"); // Invalid statement , because use Brackets