Does echo() Accept numbers?

Question

I'm relatively new to programming and am still unsure of myself when reading language documentation. The description of PHP's echo from php.net is:

void echo ( string $arg1 [, string $... ] )

Thus it seems to me that the input parameter (s) must be of type string. However, echo works with numbers. For example, the code...

<p><?php echo 3; ?></p>

...successfully prints 3 to the page. Then shouldn't the parameter type for $arg1 and $... be the pseudo-type mixed (instead of just string) to show that echo will accept strings or numbers? Otherwise, how would I be able to infer from the documentation that number parameters are acceptable?

Answer 1

echo will cast its arguments to a string.

Answer 2

Then shouldn't the parameter type for $arg1 and $... be the pseudotype mixed (instead of just string) to show that echo will accept strings or numbers?

Actually in the manual, string is sort of a pseudo-type as well. Not a real pseudo-type like mixed is (see Pseudo-types and variables used in the PHP documentation).

To be more precise, string is loosely typed:

PHP does not require (or support) explicit type definition in variable declaration; a variable's type is determined by the context in which the variable is used. [highlight by me] That is to say, if a string value is assigned to variable $var, $var becomes a string. If an integer value is then assigned to $var, it becomes an integer.

As this is for variables, the same applies to other types of expressions: an expression's type is determined by the context in which the expression is used.

In your case, the expression 3 is used in the string context of the echo function. PHP has no problem at all to use 3 as string, so you don't get an error and it's displayed (as string).

Otherwise, how would I be able to infer from the documentation that number parameters are acceptable?

echo expects string parameters. When you pass a (variable) expression that is a string, a number, a boolean, NULL or a Resource (see Types), all these types are used as strings. So whenever you see string as the type, just use a string expression. It does not mean that you need to explicitly define an expression to be string to make it work, as PHP does not have explicit type definition.