Add text at the benining of a specific line in PHP - php

Question

I would like to know how can I add text at the begining of a specific line in a txt file using PHP.
For example line 2 and 4:
Line 1
Line 2
Line 3
Line 4
to
Line 1
Whatever Line 2
Line 3
Whatever Line 4
Edit: The content of each line is variable all time, so I can't use replace or search for a specific word.
Thank you :)

Answer 1

Get the contents of the file, with each line as an index of the returned array, using file():
$lines = file('path/to/your/file');
Then you can do whatever you need by using the correct line index:
// prepend content to line 2:
$abc = 'abc' . $lines[1];
// append content to line 4:
$xyz = $lines[3] . 'xyz';
The whole process (get the contents, update them, and then replace the original file):
$file = 'yourfile.txt';
$lines = file($file);
$lines[1] = 'xxx' . $lines[1]; // prepend content to line 2.
$lines[3] = 'yyy' . $lines[3]; // prepend content to line 4.
file_put_contents($file, implode('', $lines));"

Answer 2

If you want to add every second line use this code
$n = 0;
for ($i = 1; $i <= 10; $i++) {
if($n % 2 == 1) {
echo "Whatever Line: ".$i."<br>";
} else {
echo "Line ".$i."<br>";
} $n++;
}
but if you want add only second and 4th line use this code.
for ($i = 1; $i <= 10; $i++) {
if(($i == 2) or ( $i == 4)){
echo "Whatever Line: ".$i."<br>";
} else {
echo "Line ".$i."<br>";
}
}