diff --git a/modules/lagoon/validate_rules.go b/modules/lagoon/validate_rules.go
index 63e02ec..79531ec 100644
--- a/modules/lagoon/validate_rules.go
+++ b/modules/lagoon/validate_rules.go
@@ -618,8 +618,11 @@ func phpScalarString(value any) (string, bool) {
 	return "", false
 }
 
-// phpFloatString formats a float as PHP 8 casts it to string: the shortest
-// round-trip digits, in exponent form (1.0E+15) from 1e15 up and below 1e-4.
+// phpFloatString formats a float as PHP 8 casts it to string: %.14G with
+// the default precision ini of 14 (zend_gcvt). The value is correctly
+// rounded to 14 significant digits and trailing zeros are dropped; it is
+// written in exponent form (1.0E+15, 1.5E-5) when the decimal point would
+// sit more than 14 digits right or more than 3 zeros left of the digits.
 func phpFloatString(f float64) string {
 	switch {
 	case math.IsNaN(f):
@@ -634,21 +637,38 @@ func phpFloatString(f float64) string {
 		}
 		return "0"
 	}
-	e := strconv.FormatFloat(f, 'e', -1, 64)
+	const precision = 14
+	e := strconv.FormatFloat(math.Abs(f), 'e', precision-1, 64)
 	mant, expStr, _ := strings.Cut(e, "e")
 	exp, _ := strconv.Atoi(expStr)
-	if exp >= -4 && exp < 15 {
-		return strconv.FormatFloat(f, 'f', -1, 64)
+	digits := strings.TrimRight(strings.Replace(mant, ".", "", 1), "0")
+	if digits == "" {
+		digits = "0"
 	}
-	if !strings.Contains(mant, ".") {
-		mant += ".0"
+	decpt := exp + 1
+	var out string
+	switch {
+	case decpt < -3 || decpt > precision:
+		m := digits[:1] + ".0"
+		if len(digits) > 1 {
+			m = digits[:1] + "." + digits[1:]
+		}
+		sign := "+"
+		if exp < 0 {
+			sign, exp = "-", -exp
+		}
+		out = m + "E" + sign + strconv.Itoa(exp)
+	case decpt <= 0:
+		out = "0." + strings.Repeat("0", -decpt) + digits
+	case decpt >= len(digits):
+		out = digits + strings.Repeat("0", decpt-len(digits))
+	default:
+		out = digits[:decpt] + "." + digits[decpt:]
 	}
-	sign := "+"
-	if exp < 0 {
-		sign = "-"
-		exp = -exp
+	if f < 0 {
+		out = "-" + out
 	}
-	return mant + "E" + sign + strconv.Itoa(exp)
+	return out
 }
 
 // phpTrim trims the characters PHP's trim() does.
diff --git a/modules/lagoon/validate_rules_test.go b/modules/lagoon/validate_rules_test.go
index 6af7ac1..f5b6e0f 100644
--- a/modules/lagoon/validate_rules_test.go
+++ b/modules/lagoon/validate_rules_test.go
@@ -2,6 +2,7 @@ package lagoon
 
 import (
 	"encoding/json"
+	"math"
 	"reflect"
 	"strings"
 	"testing"
@@ -209,3 +210,45 @@ func TestValidateRulesMatchLaravel(t *testing.T) {
 		})
 	}
 }
+
+// TestPHPFloatStringMatchesPHPCast pins phpFloatString to PHP 8's (string)
+// cast of a float, which formats with the precision ini (14 significant
+// digits, %.14G): the wants are php -r '(string)$x' output.
+func TestPHPFloatStringMatchesPHPCast(t *testing.T) {
+	for _, tc := range []struct {
+		in   float64
+		want string
+	}{
+		{0.1 + 0.2, "0.3"},
+		{1e14, "1.0E+14"},
+		{1e15, "1.0E+15"},
+		{1e20, "1.0E+20"},
+		{123456789012345.0, "1.2345678901234E+14"},
+		{12345678901234.0, "12345678901234"},
+		{1.5e-5, "1.5E-5"},
+		{0.0001, "0.0001"},
+		{0.00012345678901234567, "0.00012345678901235"},
+		{-2.5, "-2.5"},
+		{3.0, "3"},
+		{1.0 / 3, "0.33333333333333"},
+		{2.0 / 3, "0.66666666666667"},
+		{-1e-10, "-1.0E-10"},
+		{99999999999999.9, "1.0E+14"},
+		{0.1, "0.1"},
+		{999999999999999.0, "1.0E+15"},
+		{5e-324, "4.9406564584125E-324"},
+		{1.7976931348623157e308, "1.7976931348623E+308"},
+		{100.0, "100"},
+		{1234.5678, "1234.5678"},
+		{-0.000099999, "-9.9999E-5"},
+		{0, "0"},
+		{math.Copysign(0, -1), "-0"},
+		{math.Inf(1), "INF"},
+		{math.Inf(-1), "-INF"},
+		{math.NaN(), "NAN"},
+	} {
+		if got := phpFloatString(tc.in); got != tc.want {
+			t.Errorf("phpFloatString(%v) = %q, want %q", tc.in, got, tc.want)
+		}
+	}
+}