How can I determine whether a string exists in txt?

For example, the js.txt content in txtis 13600000000137888888881366666666618999999999 in a string. I want to determine whether $ mobile = & quot; 13788888888 & quot; exists in the txt file. $ s = file_get_contents (how does 'J determine whether a string exists in the txt file?
Js.txt content is


13600000000
13788888888
13666666666
18999999999

I want to judge

$ Mobile = "13788888888 ";

Whether the txt file exists




------ Solution --------------------
$S‑file_get_contents('js.txt ');
If (strpos ($ s, $ mobile )! = False) echo 'exist ';
Else echo 'Nonexistent ';