Can linux cat command be used for writing text to file?

That's what echo does:

echo "Some text here." > myfile.txt

Sounds like you're looking for a Here document

cat > outfile.txt <<EOF
>some text
>to save
>EOF

Here's another way -

cat > outfile.txt
>Enter text
>to save press ctrl-d

For text file:

cat > output.txt <<EOF
some text
some lines
EOF

For PHP file:

cat > test.php <<PHP
<?php
echo "Test";
echo \$var;
?>
PHP

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