PHP Manual: Set string used for indenting

Object-oriented style

public bool XMLWriter::setIndentString(string $indentation)

Procedural style

bool xmlwriter_set_indent_string(XMLWriter $writer, string $indentation)

Sets the string which will be used to indent each element/attribute of the resulting xml.

writer: Only for procedural calls. The XMLWriter instance that is being modified. This object is returned from a call to xmlwriter_open_uri or xmlwriter_open_memory.
indentation: The indentation string.

Returns true on success or false on failure.

Version	Description
8.0.0	`writer` expects an XMLWriter instance now; previously, a resource was expected.

Note:
The indent is reset when an xmlwriter is opened.

XMLWriter::setIndentString