ltrim函数不能按预期工作？ PHP

I am using ltrim function. I run this code :

echo ltrim("looooooooooooooooooooooooongWord","long");

i expect to get this word : looooooooooooooooooooooooongWord but it prints Word.

what's the problem ?

Edit: I thought it remove the word long, but it appears it remove the l,o,n,g characters !

ltrim — Strip whitespace (or other characters) from the beginning of a string

It does exactly as it says, strips chars l, o, n, g.

From the docs: (please read them).

string ltrim ( string $str [, string $character_mask ] )

str

The input string.

character_mask

You can also specify the characters you want to strip, by means of the character_mask parameter. Simply list all characters that you want to be stripped. With .. you can specify a range of characters.