Improve XML element creation and add tests for special character handling (eq. &)

src/XlsxFastEditor.php

@@ -1028,13 +1028,20 @@ public function _makeNewSharedString(string $value): int
 		}
 
 		try {
-			$t = $dom->createElement('t', $value);
+			// First, we create an empty element t
+			$t = $dom->createElement('t');
+			if ($t === false) {
+				throw new XlsxFastEditorXmlException('Failed to create <t> element');
+			}
+			// Add content as a text node
+			$textNode = $dom->createTextNode($value);
+			if ($textNode === false) {
+				throw new XlsxFastEditorXmlException('Failed to create <t> text node');
+			}
+			$t->appendChild($textNode);
 		} catch (\DOMException $dex) {
 			throw new XlsxFastEditorXmlException('Error creating <t> in shared strings!', $dex->code, $dex);
 		}
-		if ($t === false) {
-			throw new XlsxFastEditorXmlException('Error creating <t> in shared strings!');
-		}
 		$si->appendChild($t);
 		if (!($dom->firstElementChild instanceof \DOMElement)) {
 			throw new XlsxFastEditorXmlException('Invalid shared strings!');

src/XlsxFastEditorCell.php

@@ -307,10 +307,17 @@ public function writeFormula(string $value): void
 			throw new XlsxFastEditorXmlException("Internal error accessing cell {$this->name()}!");
 		}
 		try {
-			$f = $dom->createElement('f', $value);
-			if (!($f instanceof \DOMElement)) {
+			// First, we create an empty element t
+			$f = $dom->createElement('f');
+			if ($f === false) {
 				throw new XlsxFastEditorXmlException("Error creating DOMElement of formula for cell {$this->name()}!");
 			}
+			// Add content as a text node
+			$textNode = $dom->createTextNode($value);
+			if ($textNode === false) {
+				throw new XlsxFastEditorXmlException("Error creating text node of formula for cell {$this->name()}!");
+			}
+			$f->appendChild($textNode);
 		} catch (\DOMException $dex) {
 			throw new XlsxFastEditorXmlException("Error creating formula for cell {$this->name()}!", $dex->code, $dex);
 		}

tests/test.php

@@ -119,6 +119,10 @@
 	$xlsxFastEditor->writeInt($sheet2, 'C3', -7);
 	$xlsxFastEditor->writeFloat($sheet2, 'D3', 273.15);
 
+	// Writing special XML characters
+	$xlsxFastEditor->writeString($sheet2, 'B5', '< " & \' >');
+	$xlsxFastEditor->writeFormula($sheet2, 'C5', '=LEN("< & \' >")');
+
 	// Writing non-existing cells but existing lines
 	$xlsxFastEditor->writeFormula($sheet2, 'I2', '=7*3');
 	$xlsxFastEditor->writeString($sheet2, 'F2', 'γ');
@@ -166,6 +170,10 @@
 
 	assert($xlsxFastEditor->readString($sheet1, 'B2') === 'World');
 
+	// Test special XML characters
+	assert($xlsxFastEditor->readString($sheet2, 'B5') === '< " & \' >');
+	assert($xlsxFastEditor->readFormula($sheet2, 'C5') === '=LEN("< & \' >")');
+
 	$xlsxFastEditor->close();
 
 	// Verify by hand that the resulting file opens without warning in Microsoft Excel.